The Cavalier fighter (Xanathar's Guide to Everything, p. 30) gets the Unwavering Mark ability which lets you mark a creature and states (emphasis mine):
In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. [...]
Regardless of the number of creatures you mark, you can make this special attack a number of times equal to your Strength modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.
In most cases, it would be fairly easy to figure out how many of these special attacks the Cavalier can make, because the Cavalier's Strength usually does not fluctuate. But a lot of things can happen during a busy adventuring day, for example, Strength damage by some nasty monster, some kind of transformation magic or the ingestion of mystical potions. All these things could temporarily alter the Cavalier's Strength. So my question is:
If the Cavalier's Strength changes, does the number of special attacks against marked creatures change as well?
I assume the answer is Yes, but I'm not sure how to handle it if the Cavalier's Strength increases or decreases for just a little while.