Okay, first things first. You talk about knights but the definition you provide is for a mounted warrior. The term you're looking for is a man-at-arms. A knight is always a man-at-arms but far from all men-at-arms were knights. Generally speaking the somewhere between 10 to 25% of men-at-arms were knights, depending on time period and campaign. Functionally they are the same on the battlefield but knighthood is an institution, a man-at-arms is a type of soldier with heavy armor, horse, lance and sword.

So how much support would they need? Well, that depends. If you were to equip a man-at-arms out of your own pocket it would cost you around 11 pounds in the 15th century. This is the cost of an off the shelf harness (£6 for a plate armor that is not bespoke) and a semi-decent fighting horse (£5 on the low end). I'm assuming here that the man in question already owns at least a riding horse and a sword. If he doesn't, add 2-3 pounds. To put this is perspective, it would take a journeyman about 15 months of labour to earn the same amount of money.

Added to this a man-at-arms could expect about a shilling per day while in the field so add another £ per 20 days of service. So, if the man-at-arms wanted to purchase his own equipment it would take him about 7 months on campaign to earn the cost of his armor and horse, provided he had no other sources of income and no expenses (which was of course rarely the case).

Now, estimating tax burden i medieval europe is not my forte so I won't attempt to translate this into modern terms but his should give you a rough idea of how much it cost to field a man-at-arms. As for Novogorod, I don't know enough about the region and time period to say but it does not strike me as unreasonable, a similar system was used in Sweden from the 16th century and onwards. Based on the cost above, if 10 men were sharing the cost it would come to about 15% effective tax rate. Now, whether or not this is reasonable or not depends on the country, time period and the cost of equipping the soldier but under the Swedish allotment system a cavalryman usually required only one or two freeholding farmers for support (in exchange for which they were extempt from taxation and military service) whereas an infantryman could be supported by about 4 of what can roughly be compared to crofters or smaller freeholding farmers.

Now, I should clarify that this is not how it worked it practice. Generally speaking a subject was bound by law to provide a certain number of men of a certain type to his lord, how he did that varied but typically he either supplied the men himself by paying and equipping them or he paid a roughly equivalent sum of money for someone else to raise them or hire them as mercenaries.