Sounds like a silly question. 150 guests = 150 invitations, right? Actually, you should order by household, not by headcount. But there are other factors you should take into consideration before placing your order.
Calculating Your Quantity:
1 per household
3-4 for keepsakes
Order a few extra so you have pristine samples to store in your scrapbook and for parents/grandparents to preserve.
1-2 for your photographer
Invitations set the tone for your dream day. Your photographer will want to include some shots.
10-15 extra invitations
You never know when mail might get lost or there are last-minute additions to your guest list. It’s much less expensive to include extra invitations with your original order than to do a separate order later.
25 extra envelopes
Of course, you need additional envelopes for the invitations. But you should also add in a few more in case of addressing errors.