Like any Planeswalker entering the battlefield, they will do so with a number of loyalty counters shown in the lower right corner
There is an argument to be made about the Final Aeon possession also being part of this mechanic, but I am not really so sure
These combine to form a 40-card deck, and you keep the cards (about 22 non-basics) as a reward
Scavenging Ooze actually sees a ton of Modern play, and Id grab a set at $3 retailits a bargain
However, if you can't use them to keep your opponent down and they get a foothold in, you're going to find it very difficult to claw your way back
During later stages of design, we do a pass on all the cards that create tokens to see if we can consolidate them so players need fewer individual tokens