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.

If, for some reason, the creature 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.

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 is countered, the squad ability will not trigger, and no tokens will be created.

If the spell resolves but the creature with squad leaves the battlefield before its squad ability resolves, you'll still create the token copies.