SQL Server 2012 Performance - Reads per second
The question itself is too nebulous. You asked how many Selects per second the database can handle? Well, can handle before what happens? What level of performance is acceptable? Do you need results returned in milliseconds, seconds, minutes? What is acceptable? How many rows are being returned from these Selects? What does the query plan look like? 

Ultimately no one here is going to be able to tell you what you want to know. There are too many details you didn't tell us. But you can certainly find out for yourself. It's not at all difficult to stress test your database, to find this out for yourself. If you are a developer, you could easily write a C# program that creates threads that performs these Selects. It would be quite easy to measure performance as you gradually increase the number of threads. If you are not a developer, I have to believe there are stress test apps you can buy, or find open source, that will enable you to measure this. If determining a rough idea of how many users your database can accept is that important, you need to be doing this anyway.