the 250 watt will be great.
I don't know how many plants you should grow. That all depends what kind of growing style your into.
IMO: Since it is your first grow I would just grow a few plants under that light and start flowering when the are about 20-25cm tall.
They are going to stretch when you start flowering, they will double in size, some strains will even triple there size the first two weeks of 12/12
make sure to keep that in mind
Keeping it simple is always best, especialy for a first grow. You want this to be fun so it helps inspire you to keep growing.
just for fun you can search:
SOG, SCROG, LST

peace