Very hard to tell without exactly knowing your habits and usage of stuff. You can check your internet usage in windows or on your phone to get a rough idea of which apps use how much. If you are careful and stop yourself from doing some things you can manage, but it might be pretty tight, it’s only ~2gb per day.

You will need to monitor how much you are using because it’s very easy to do something that unexpectedly uses lots of data. Setting a daily data limit on your phone is a good idea. For example, set a daily 2gb limit (or slightly less, to be safe), it will notify you when you are approaching your limit so you can adjust your usage for the rest of that day, so you don’t run out before the end of the month.

It will significantly limit what you can do, downloading games or programs will deplete your data real quick. Things like major windows updates can be like 5 gb, that’s 10% of your montly limit already for example.

Disable automatic updates for as many things as you can, especially things like PC games, windows updates, etc. If you sometimes have access to an another free/unlimited source of internet try to do them only when connected there.

Outside of downloading apps and games, the biggest usage will most likely come from watching videos and looking through social media sites with thousands of images/videos that you are constantly downloading as you scroll, so you might need to use them less, watch videos in a worse quality etc.

Watching videos takes a huge amount of data, way more than playing online games. Downloading and updating games does use a lot but just playing them barely uses anything in most cases.

The rest of the space must be unallocated, so you don’t see it.

Open up disk managment by right clicking the start menu (windows logo) and choose Disk Management.

There, on the bottom you should see your ssd and its partitions. Right click on the unallocated space, and choose New Simple Volume. Then just click through all the options, no need to change anything.

The next time you open the file explorer your new partition should be there.

You could also extend your C partition to have the entire size of your ssd but having a separate partition for the system is better. This way you now will have a 200gb system partiton that you can format to reinstall your system without losing any data on your second, ~800gb partiton.