Jet/ACE has a hardwired limit of 255 connections.
But you'll never get close to that in real life.
How many users can share your data simultaneously depends on what they are doing. If you have lots of read-only users, you'll be able to support more simultaneous users than if everybody is adding/editing all the time.
For an app with mostly read-only and some editors, you could pretty easily get 100 simultaneous users, but the more editing users, the fewer you'll be able to manage without jumping through hoops.
To me, the practical limit is somewhere in the 15-25 range. That's the point at which I'd start the process of upsizing to a server back end. It's not that you can't do better than that, it's just that to support more simultaneous users than that, you start having to be very careful with how you interact with the data, and you might need to go to unbound data editing forms. Once you've done that, you've given up 75% of the advantage of an Access front end.
As to the "splitting" issue, it's not so much a matter of how many simultaneous users, it's just a matter of not working at all if you don't split. Any Access app with more than one user needs to be split. No exceptions.
That way there's one back-end data file on the server, shared by all, and all users have an individual copy of the front end application on their workstations, with linked tables pointing to the shared back end stored on the file server.
In regard to connections, don't worry about that. An Access app with a Jet/ACE back end is more an issue of users, as you'll use a single persistent connection. It's counterproductive to try to manage connections, opening and closing them, as it taxes the file sharing mechanisms entirely unnecessarily. That is, you don't really gain much of anything in capacity, but you end up using up a lot of CPU cycles and bandwidth for no useful purpose.
This is, of course, heretical advice in comparison to client/server programming practices, but an Access app with a Jet/ACE back end isn't client/server, so you shouldn't be worrying about that.