We don't really have a good sense of how many dresses a woman of any age would have owned, or how often they would have been updated. The former would probably take a lot of dedicated reading of wills and inventories that typically isn't done in this period (it's a very Early Modern avenue of research) and is, in any case, full of issues in and of itself - women often didn't make wills, and clothes that they owned at the time of their deaths doesn't necessarily correlate with clothes they were regularly wearing, etc. And likewise, there are few to no sources that could give us comprehensive quantitative results about dress remodeling.

What we can say is that the attitude toward clothing circa 1800 was very different than attitudes in the present day. Clothing was inherently valuable in a way that it certainly isn't anymore, making up a huge portion of family expenditures, just under rent and food. In part, this was because one had to buy the fabric and then take it to someone else and pay them to do the sewing, but the main issue was that fabric was quite expensive. The album Barbara Johnson kept of the fabrics she purchased for clothing from 1746 to 1821 gives us a fairly good sense of her expenditures, and the amount of new clothing she might have in a year; the fabric she bought ranged from 20 pence to 11 shillings per yard, and could total up to 4 pounds 12 shillings for one garment! The more expensive fabrics came earlier in her life, when rich damasks and brocades were in style, and by the period you're asking about, she tended to be purchasing the cheaper calicoes, muslin, and sarcenets that were fashionable, generally about 2-5 shillings per yard and 6-12 yards per gown.

(Barbara Johnson's album can offer us some insight into the question you've asked: she typically had one to three new garments per year throughout her lifetime. But there's no telling how long she wore them or when and how she had them updated - she just didn't note that down.)

Not only was clothing very valuable, it sent important messages about the wearer, and the most important message was the wearer's status. Of course, this could be done with obviously expensive silks and lavish trimming ... but the ability to be up-to-date was just as strong a signifier, and had the ability to also show that the wearer was in touch with what was in style. At the time, there was no concept of wearing "vintage" or of showing one's quirky individuality through eclectic choices: the important issues were being fashionable in such a way that was flattering to the wearer.

As one of the pre-eminent matrons in the community, Mrs. Bennet would have found it very important to dress in such a way that showed her status. Contrary to popular belief, it was married women who would tend to be the best-dressed in a family, not their daughters: eligible young women generally presented themselves as delicate and demure in pale colors and light jewelry, while married women could have much more splendid gowns and the family's heirloom jewels - they were the ones with the money and agency to imitate the fashion magazines. Lady Catherine de Bourgh would have likely been even more intent on being the best-dressed woman in the room, as it helped to bolster her opinion of herself as the most important woman in the room.

There's a stereotype in the reenactment/online fashion history community that older women "clung to the styles of their youth," but there is actually very, very little evidence of this. The clothing in dated portraiture of older women is pretty much always in line with the given dates. It's a case of presentism, applying the post-1950s standard of a powerful youth culture and more individuality in dress into the past.

The 18th century revolutionized fashion concepts as well as economic, political, and philosophical ideals. The stiff, formal, and elaborately ornate styles of the early 1700s gave way, by the end of the century, to simpler garb. As the century progressed, English styles included simpler garments based on pastoral life , which came into vogue in England and moved into Europe. In the late 18th century, English influence relied on a sense of propriety rather than the decadent ornamentation of the elite...

Having said that, we need to set our minds in a completely different time: people didn't necessarily need to do laundry the way we do today. In fact, it was perfectly normal to wear a fairly plain garment under the clothing that could be seen, in order to protect it from the sweat and oil of the body. This goes back to the AngloSaxons. Men's shirts and women's shifts of the 18th century were made of linen, cut in fairly basic geometric shapes that made use of the woven selvage of the fabric to form unfrayable straight lines, and sewn together with firm stitches that would hold up to repeated rough laundering. Linen of very different qualities we used here, from very fine and almost sheer cloth to unbleached tow that would have to be broken in somewhat through both laundering and repeated wear to be comfortable.

In the early 19th century, cotton began to edge out linen as, very basically, cotton production was overtaking it. Just to mention some cases the American textile mills of this period were pretty much set up to deal with cotton, and the ones in England were going that way too. Cotton then became cheaper than linen so it became the preferred fabric for undergarments. Elizabeth Gaskell wrote of a woman dealing with "the puzzle of devising how [Corazza shirts] were cut out" in the 1851 Mr Harrison's Confessions.) Chemises had various forms, with laces and even embroidered ribbons. So, though they were fancier, they were still intended to be unseen and to be replaced and washed as needed. In this case "needed"is a rather complicated term. Mainly because we dont know for sure the extant pieces of clothing, and it wasn't regularly discussed. In the early modern period, there's evidence that people began bathing less specifically because they could have more changes of linen, which took care of the issue. Throughout the 18th and 19th centuries, the ideal was to change at least every day, perhaps more often if one had gotten sweaty doing something physical or if the day were hot - having clean clothes made you more comfortable, obviously, and having crisp, fresh linen around the neckline of your clothes or peeking out from your sleeves was a symbol of both your social status and your moral standing - fiction and non-fiction of the 18th century takes note of the state of individuals' linen, drawing conclusions from the contradiction between someone's greasy, dirty shirt and other fine things they might have. Dirty linen was also thought to cause or attract illness by trapping impurities and "poisons" next to the skin. The majority of people had a decent number of changes of body linen in their wardrobes, because these items were a necessity. In late 18th century France, for instance, three-quarters of the people in the bourgeoisie and artisan classes owned between ten and thirty shirts or shifts. Garsault's 1771 Art of the Linen-maker described a noble trousseau containing 72 shifts, which would allow for multiple changes per day as well as several to wear while others were being laundered! The very poor, however, might own only two shirts/shifts, wearing one while the other aired out or sat - or just one, if they were truly destitute. Paupers were typically given an outfit containing only one shirt or chemise upon entering a charitable institution, and they would rewear them until given fresh ones; as the laundries in these places were often inadequate, the "clean" one might not be much better than the one they were already wearing. With outer clothing, there was not much objection to wearing the same thing repeatedly as there wasn't an implication that the proper laundry wasn't being done: the only issue might be that you appeared to have few items of clothing, which obviously would show your class/wealth. However, it's likewise difficult to say exactly how frequently individual coats, breeches/trousers, or gowns were worn, since inventories made after an individual's death rarely make it clear which clothing has been held onto for sentimental or other reasons, and which were actively being used. The 1747 inventory of the clothes and accessories of Mary Churchill, 2nd Duchess of Montagu, for instance, tells us a lot and a little. She had 25 shifts (probably all in regular use, since there would be little reason to hold onto worn-out ones) and 27 gowns of various fabrics, from seersucker to silk - were half for winter and half for summer? Were many held onto from her early life, or were they in regular rotation? We don't know. Barbara Johnson, an 18th century Englishwoman, kept an album with snippets of all of the fabric she purchased and had made up into clothing, usually with a note of the year and what was made, which is both useful and unuseful in a similar way: we can tell her habits of consumption, but not how often she got rid of or remodeled anything. So you see, tricky question