Is it permissible for someone who has vowed to memorize a specific number of hadiths or texts to memorize more if they find themselves capable, or must they adhere to the number they vowed?

It is permissible for one who vowed to memorize a certain number of verses or hadiths not to exceed it, to exceed what he specified; because not exceeding is not an commanded act of obedience, nor is it an act of worship by which one draws closer to Allah. Limiting oneself to a specific amount is permissible, and vowing what is permissible is not considered binding by some scholars. It is permissible to substitute the vow with something better and more rewarding.