Multi-site for user groups
Can the multisite manage multiple databases with a single software?
  


Yes.


  
  Do I have to foresee it and create it from the beginning or can I create the software and subsequently, if there is a request, transforming it into a multisite?
  


You can perfectly start with a single site, and late on transform it into a multisite.

As an alternative to using a multisite, you may also want to consider using the Group module. From the specs in your question, it appears to me that only using this module (without a multisite setup), it could well be sufficient ...