ask some questions. if you have created a forum. and you post does it consume your webspace? so if every time you post on the forum it also add in the webspace?
Of course. Everything is data and has to be stored somewhere. In this case it's stored in the database.
what if i have a web space of 350mb if i have lots of post does it consumes the 350 mb? can 350mb webspace host a forum?
It depends on how big your forum will be, how many users will register? How many users will post...
But in most circumstances, 350mb is more than enough.
what if i have around 2000 people register and about 200 000 post. does it fit in 350mb?
I know a site with almost 2000 users and 100,000 posts (not to mention private messages everywhere) that had its database backup take around 160MB. So, I would venture to say yes. Just prune old topics and you're quite safe.
Some hosts may count file and database space separately. Most of MyBB data is stored in the database. The only things that would be in the filesystem are the MyBB core files, plugins, and uploads.