There are just a few things PayPal will let you do and specifying quantities is one of them. Since you've broken down each session into its' own form, here's the perfect solution.
Add this within the form for each button (right before the image) and they'll be able to specify their desired quantities.
Code:
Qty: <input type="text" name="quantity" value="1" size="2" maxlength="2">
PayPal will calculate the price based on quantity when it gets the rest of the information in the form.