If all other factors are the same, Access will invariably come in second for any modern database if the choice is Access or an SQL server. Because the SQL server will send less data over the intranet, and network data sent per query will be accordingly reduced. Meaning that the network can handle proportionately more queries with the same bandwidth.
If you know the cardinality factor for the queries, you can compute the effect on the bandwidth. Suppose your cardinality is x. That means that you will examine 1/x of your database with each query. BTW, x will be an integer or fraction always greater than 1. By no stretch of logic at all, that equally means that the SQL server will permit x times as many queries in a given time period as an Access database on some shared server.
As to computing the crossover point, you need to do a sophisticated analysis or model that is part queuing theory, part network traffic analysis, and part crystal ball. (It's that last part that always gets you...)
You also need to know the performance of the system that is going to be the host for your database. Not just CPU performance, but disk throughput. Network capacity.
Another factor is licensing. An SQL server is designated for a specific number of users per its licensing scheme. The O/S under which that database SQL program is running may have limits to the number of connections, number of sessions, etc.
Getting help for your problem through this forum is probably not going to be enough to help you very much. Too many factors need explicit analysis by someone skilled in system analysis and design. I am sorry if this doesn't help but I am sure you understand that you have asked a non-trivial question.
