Postgresql on multiple servers

Question

Supposing a lot of user access concurrently to a postgresql database. Supposing the database is on a dedicated server which only contains postgresql. Will the database hold ? For 100 users ? 1000 ?

Another question: If i'd like to make it hold no matter how many users, is there a built-in way of separating the database onto several servers or would i have to program the separation myself by installing several postgresql servers and splitting the data ?

Edit: I mean transactions, not the size. And the queries I want to run are simple inserts and selects with joins.