This all depends on how large you expect your wedding and reception to be and your preferences. You should have one invitation made for each household you plan on inviting to the reception. Make sure to order more than you initially plan for. Having a few dozen extra invitations on hand will prove useful for guests whose invitations are lost in the mail or family/friends you may have forgotten about, but still want to include.
Smaller weddings may only require 100 invitations, while larger wedding receptions may warrant over 1000 printed.