There will be total 1.5 billion documents in nearly 100 collections in one database. Need I to separate this database into several? Is there a performance limitation for a single database when the quantity of documents going higher?
It's more like a hardware problem. Especially memory! Not CPU. So, if you can give that DB server quite much memory, like a few hundred of GB, it will be fine!
Of course, the first question is "how big" is the DB? Especially how big partition of that data is "active". And how fast that DB must serve clients.