A per SSID limit is the maximum bandwidth for a given SSID, per access point.
The calculation you mentioned above works in theory, but it assumes that your guests will be equally distributed across all available Access Points, which in the real world is very unlikely.
Additionally, 500kbps bandwidth for a given user is realistically worse than having no wifi at all.
For most websites and web applications these days it will be too show to be usable.