How effective will a window unit AC be for a garage?
It's all math.
You need 1 ton (12,000 btus) of cooling per 400 square feet.
You need about 500-600 btus of cooling per person.
So in 400 square feet you can expect to cool up to 20 people per ton assuming they aren't actively exercising. I wouldn't do any more than 10 people per ton if you want them to feel at least a little chill.
A hot and uninsulated garage will likely need 20% more btus than my calculations above.
You should turn these units on 6-8 hours before guests arrive to ensure your garage is sufficiently comfortable.