Open iTunes and go to Ringtones and delete the ones you whish. (With alt-cmd-S you will show the sidebar)
You will be asked as usual if you wish to keep the files or send them to the basket.
At next sinc they will be deleted from the iPhone too.
Ringtones should not use much memory on the iPhone and I believe you can load many but no idea of a specific limit.
To detete any media from iTunes, select the item(s) and then select "Delete" from the Edit menu. It will ask you if you want to premanently delete them from iTunes - say Yes.
As far as ringtones on the phone - I am not aware of any limit save the amount of memory you have available.