Dynamically populate quantity field based on currently logged in user

This is my situation:

We will pre register around 12 users using the Restrict Content Pro plugin, so that the form itself is login protected.

Once the user is logged in, we would like a product quantity field to be dynamically populated with a value that we have determined ahead of time, that will vary per user.

I'm not sure how to go about this. I can see that we will need to store the quantity values for each user somewhere. Or we will need some sort of if statement along the lines of
if (user = username) { quantity = number }