Rollback segments

Hi,
I have read the documentation on the rollback segments and just wanted to get a good hold of it!

If I have 1 rollback segment,with Initial 2M next 1M minextents 20 maxextends unlimited then

1) If 20 people log in simultaneously, will each get a separate extent?.
2) if there are 40 People, will the system extend once for every addition user or assign each to the existing extents?
3) Can there be two users using the same extent.
4) In the above case -Next is 1m, does that apply only if the transaction that was assigned to a extent wants an additional extent.

Please clarify
Badrinath