So far in all of my Google searches I can’t seem to find out the user limit for Sharepoint 2013 foundation. Apparently an organization was limited to having only 15 users if they had Sharepoint 2010, but I can’t find any information about 2013.
I glanced through the link below but did not find anything helpful.
http://technet.microsoft.com/en-us/library/cc262787%28v=office.15%29.aspx