In Access, or on SQL. If SQL, how many connections can you have on your SQL Server? Are you running licences with CALs, or Cores? The answer depends on your answer to that. If Cores, then as many as you want, realistically. If CALS, how many CALs do you have?

On Access, it's not recommended to have more than one user to have the same Access file open at the same time (talking from experience) if you're storing data locally. It's not really a problem if all your data is hosted on SQL, as SQL Server handles multi users well.

Your latter question is very open ended. What is your goal? Are you creating a front end interface using Access, datasheet views? I'm not sure there is really a "best practice", but personally I'm not really a fan of Access amending data, or inserting on SQL (although, I haven't done it since Access 2010).