Rulings

You may pay the squad cost any number of times. You will create a token that is a copy of that permanent for each time you paid the squad cost. If you paid the squad cost multiple times, the tokens will all enter the battlefield simultaneously.
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."
The tokens created by the squad ability aren't "cast," so any abilities that trigger when a spell is cast won't trigger for the copies.
If the spell resolves but the permanent with squad leaves the battlefield before its squad ability resolves, you'll still create the token copies.
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.
If the spell is countered, the squad ability will not trigger, and no tokens will be created.
If, for some reason, the permanent doesn't have the squad ability when it's on the battlefield, the ability won't trigger, even if you've paid the squad cost one or more times.