I am pretty sure "draw 50" refers to Lee J. Ames' "Draw 50 Somethings" series of wordless instructional books, where "something" ranged from "animals" or "athletes" or "dinosaurs" to "Beasties and Yugglies and Turnover Uglies and Things That Go Bump in the Night". Drawing fifty instances of a thing out of your head is a good exercise but completely different from those.
https://duckduckgo.com/?q=lee+j+ames+draw+50&t=osx&ia=images...
Oh cool! Yeah it made me think of these internet challenges "I'll draw and post 1 portrait a day for the next year" or whatever. And art classes "give me 20 sheep". Where it's fine to start "literal" but you are encouraged to get bored of that quickly and become more creative with it.
For me, "draw 50" series is about letting loose, NOT about skill. On the contrary, it's about exploring the space of all the different ways a prompt can be worked with in an UN-literal manner. Overall drawing facility will improve merely as a byproduct. The ability to be whimsical and exploit drawing flaws, erm, deliberately, is the real prize.
Different from "limbering up" exercizes like drawing your hand - where you do one at the beginning of a session just because it gets you started and in the flow without having to think or pick a "real" topic. (A similar idea is to leave some already decided work mid-done: when you arrive in the morning you can work on that and limber up and do something useful without having to think clearly yet.)
Different from thumbnails which are about picking a direction.