Can an SSD simultaneously function as both an OS drive and storage drive?

I'll be ordering a Sager NP9377 soon and I want to know if I can still install programs on an SSD that has been configured as an OS drive. If I order the laptop with one SSD configured as an OS drive will I need a second SSD for programs and games or can I have the laptop preconfigured with only one SSD for the OS and also install more programs and games on it?

Also, just curious, how much storage space does an OS use on an SSD? I'm considering getting the 256GB Micron M600 SSD with a 1TB...

Can an SSD simultaneously function as both an OS drive and storage drive?