maximum allocate RAM for MS SQL Server I have MS SQL Server with different size of RAM.
So I would like to control the values which are set for the RAM.
Somewhere I read a 'rule' to calculate the maximum of RAM.

These SQL Server are only for DB's. At the server itself is nothing else running.
Only two remote session are possible.

How many RAM does the OS and the two remote session require?
Calculation is something which you need to analyse an figure out with some calculation on basis of that. Kindly go through the below discussion which will help you out in deciding what values you can assign:

What is a deterministic method for evaluating a sensible buffer pool size?

Also, would recommend to go through below article for indepth explanation:

http://sqlblog.com/blogs/jonathan_kehayias/archive/2009/07/07/understanding-the-vas-reservation-aka-memtoleave-in-sql-server.aspx