How many dresses would queens and high-ranking duchesses have owned? Did kings and elite lords re-wear their best garments on state occasions, or was it uncouth to be seen in the same clothes? Were clothes worn until they wore out, or handed down? To whom? Was there competition to make e.g. the future queen's wedding dress?
tl;dr I know nothing and I am curious.
(I'm sticking to Elizabeth I and Marie Antoinette, as they're the only ones I know much about, sartorially.)
It's very hard to know how much they owned, because they so often gave them away, especially to servants who worked in their wardrobes or dressed them, as did non-royal and non-titled women. Though "servant" in the case of queens encompasses ladies-in-waiting. This was usually done before they wore out, both because it was important to be seen at one's peak at court and because it was important not to insult the person by giving them an unwearable item.
We know from an inventory done of Elizabeth I's wardrobe around the time of her death that she had accumulated 67 round gowns, 100 loose gowns, 102 French gowns, 99 robes, 127 cloaks, 56 outer skirts, 85 doublets, 125 petticoats, 126 kirtles, 136 stomachers, and 18 lap mantles. (One source I've found suggests French gowns are trained while round gowns are cut to a round length, ie the same all around.) Mme Campan's 1820s memoirs of court life with Marie Antoinette describe the queen as having 36 gowns new every season (except maybe spring was reused for fall?) - 12 formal court gowns, 12 informal robes de fantaisie for the afternoon, and 12 formal-but-not-court-gowns for the evening. These aren't the best sources, because the former could include clothing that's been saved for Reasons as well as what Elizabeth had been wearing more recently, and the latter is of course the dreaded memoir written from a distance (and Henriette Campan definitely wanted to help rehabilitate MA's reputation), but they're the best we have.
I'm not sure about never-wearing-it-twice rules. There are a lot of uncited 19th and 20th century references to those both historically (and contemporaneously, wrt Empress Eugenie or fashionable women in general, nearly always in a satirical or sarcastic tone, so they're all suspect to me), but nothing concrete that I'm aware of, though maybe /u/kittydentures knows and will set me straight. For Elizabeth, giving and being given expensive clothing was a big deal, and MA essentially had people helping themselves to her possessions as a perk, so high turnover/expenditure was a constant. But I'm not sure we really know if there were unspoken rules about never wearing the same gown twice. With Campan's figures, MA would certainly not have been able to only wear each gown once.
I don't believe there would have been competition to make a royal wedding dress, if only because you generally had a dressmaker to the royal family who would automatically get it.