can we assign multiple role to a single user or client.

I strongly suggest NOT having a Role per store. There is a much better way to do that using an extra table and just one Role.

QuickBase will give th Maximum access possible based on all the Roles a useri is in.

iI suggest that you post a new question being clear about the access level you are looking for by Role, for example in your post it was not clear if managers would have access to all stores or just some stores.