Access statistics have a maximum user count of 255 on a Jet back-end
database. I've used up to approximately 75 concurrent users, but many of
them were reading, not writing. I seriously doubt that you can have that
many users without having some corruption problems. OTOH, you should be able
to have that many Access front-end users on a SQL-Server back-end without
any difficulty.

When you say "splitting the data", do you mean creating separating the data
from the application?  If so, this is a must for any multi-user Access
appliction.  NEVER allow multiple users to use the same mdb file sitting on a
network.  They should all have identical front ends and all point to (have
tables linked) from the backend (data) sitting on your server.

As Arvin stated, you should probably check into SQL Server Express (if you
don't expect and individual database to exceed 4 Gig), because it is free for
download.  If you expect your data to exceed that, you could probably segment
the data into separate SQL Server "databases" and still link to those in
Access, I just don't know how efficient it will be to create queries that use
tables from multiple SQL Server "databases".  The other option is to step up
to SQL Server or go to an Open source database like MySQL.