How much cash in a toonie bar float?

I would carry a $500 float. Cash carriers will most likely have $20s on them so you should have at least $300 worth of $5 bills (that's only 60 bills) and $200 worth of loonies. Hopefully some guests will have change and the people paying a $20 will return with a $5 you gave in change so you'll accumulate the small denominations back.

Are you running the bar yourself or is the caterer doing it? If you're doing it, you'll need a Special Occasion Permit and a licensed server. You can then buy alcohol using that permit at the LCBO and any unopened bottles can be returned after the wedding. That way you don't have to feel like you're overbuying.