Hi,
The limits of 3CX are based on 2 things.
1. Your call licence
2. Your machine.
3. Your network.
All 3 things can be inproved, you can have as many PSTN devices/ voip providers or extensions connected. Having said that you can inmagine the enormity of netwrok traffic that can be generated when making a lot of calls, or those done by the re registration period of each device.
Before you go on a PSTN buying spree, find out if there is a more pratical approach as to what you would like to achieve.