Multiple wp installs would use different tables. wp_1 or wp_2 etc, to keep them separate from conflict. I’d not use the wp_ prefix myself, but use different two letter/number combinations to avoid hacking, but you can have lots of installs on one db.
Now there might be some technical reason to avoid that, such as speed, and all, but that’s something a techie would have to answer. I don’t know of any ‘limits’ being mentioned by WP on how many installs per db.