Cita: Microsoft Access Database - General
General specifications for Microsoft Access databases Attribute Maximum
Microsoft Access database (.mdb) (Microsoft Access database: A collection of data and objects (such as tables, queries, or forms) that is related to a particular topic or purpose. The Microsoft Jet database engine manages the data.) file size 2 gigabytes minus the space needed for system objects (system object: Database objects that are defined by the system, such as the table MSysIndexes, or by the user. You can create a system object by naming the object with USys as the first four characters in the object name.).
Number of objects in a database 32,768
Modules (including forms and reports with the HasModule property set to True) 1000
Number of characters in an object name 64
Number of characters in a password 14
Number of characters in a user name or group name 20
Number of concurrent users 255
http://www.databasedev.co.uk/access_specifications.html
Ahora, por experiencia puedo decirte que independientemente de los usuarios, cuando las consultas son sobre tablas con ya algunos miles de registros, lo vas a querer aventar por al ventana y poner SQL Server MSDE, SQL Server Express, Oracle Express, InterBase o incluso MySQL. Aunque mucho tiene que ver tu programación, claro esta.
Saludos