I have a razer mamba TE however I don't know what my DPI and sensitivity (in-game) should be for CS:GO. The pro players have a DPI around 400 and a sensitivity of 1 to 2 however when I apply their config, the sensitivity is way too low. Used to have a very old mouse which didn't have a custom DPI and my sensitivity for CS:GO was 8. What should my DPI and sensitivity be?

Pro players play on an eDPI of around 1200-800, generally a sens value of either 1 or 2 (I'd recommend 1).

Pros play with a BIG mousepad and make big mouse movements. You can certainly do the same if you want to. 800 dpi with 1 sens is good. Make sure to set your windows sensitivity to 6, and disable mouse accel and enable raw input.

It will feel slow until you get used to it, then it will feel fine.

i use 1000 dpi and 1 in game sense
it's entirely preference once it goes below that

tbh the dpi and sense is up to your own preference. i have a friend who's running up to 4200 dpi and 3-5 in sense, and he's able to pick headshots off one after one. he has just gotten used to it and feel comfortable with it.
i find it easier with low sense and a bigger mousemat. i'm running 800dpi and about 0.8 sense ingame.
choose what you're comfortable with.

the space i have is like 4 hands in size so yeah i've got like 2000 dpi and 3 ingame sens

800DPI/0.9sens checking in.

Get a large mousepad. It'll work wonders. Although your arm may get tired rather quickly.

I use the same as box, but I have an extended razer mouse mat which I cannot recommend enough.

pretty much the opposite

1800/1.5 Arm pretty much still. It's fast to others, but it's the way I play. Play what you feel comfortable at. Just because the pros play slow, doesn't mean you have to. But generally a low dpi and sens are what you want.

Don't copy others. Ramp the DPI right up, and lower the sensitivity so you can get >360 rotation without having to lift the mouse - depending on the available space you have this will vary (giant pad recommended). Test. Adjust. Repeat until it feels right.

When would you ever need to do a full 360? Unless you're making a no scope montage you'll only ever need to one half spin. As you start to play against better people, this matters even less because if someone is behind you, you're probably dead.