If your ambition therefore be to equal Raffaelle, you must do as Raffaelle did; take many models, and not take even him for your guide alone to the exclusion of others. And yet the number is infinite of those who seem, if one may judge by their style, to have seen no other works but those of their master, or of some favourite whose manner is their first wish and their last.
I couldn't understand why the number is infinite? Because the number just refers to the work numbers of their master. In my view, a master's works are limited, no doubt.
No, 'infinite' just means 'many'-- there are many artists who have seen only the works of their masters or a favorite artist, and no works of other artists. The writer decries this attitude.