If enough -1/-1 counters are put on a creature you control at the same time to make its toughness 0 or less (or the damage marked on it to be lethal), the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many Mutagen tokens you get. For example, if there are three +1/+1 counters on a creature and it gets four -1/-1 counters, you'll get three Mutagen tokens. That's because The Ooze's triggered ability checks the creature's existence just before it leaves the battlefield, and it still has all those counters on it at that point.

Mutagen tokens are a kind of predefined token. Each one is a colorless artifact with the artifact subtype Mutagen and the ability "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery."

Some spells and abilities that create Mutagen tokens require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. The Mutagen token won't be created.