For streaming you should use the integrated nvec enconder of the RTX 2060 (
https://www.nvidia.com/en-us/geforce/guides/broadcasting-guide/). While using this encoder you should have no impact in your gaming performance at all.
As for how many FPS you will get it all depend on the settings and how you build the computer.
Theres no doubt the GPU and CPU are the most important parts of the FPS equation, but RAM is also a key aspect of the final number.
For Ryzen you will want to get atleast 2x8GB DDR4 3200MHz CL16 or 2x8GB DDR4 3600MHz CL16 (just so you know, as long as the memory kit you choose have really decent timmings the diference will be small between DDR4 3200MHz and 3600MHz, but the 3600MHz kit may give you a tiny FPS advantage).
I don't play a lot of CS:GO, I haven't used the game since I updated it my rig last year, but from what I know you should get AVG FPS really close to 250, who knows?, maybe depending on the graphic settings you can go way higher than that.
Also keep in mind you can play the game at for example 1080p, and stream at 720p. The RTX 2060 Super should eat CS:GO like a sweet pancake.
Cheers