Hi Acorn!
Well... legally I think you can have 5 per acre (where I live), but it depends on where you are. I'd check with your county and/or conservation district about how many per acre are allowed where you are.
The conservation district are very helpful. I would get in touch with them - they'll actually come out and talk to you and answer questions and look at your property! They're AMAZING and will come for free!!
I'd say you could have 2 alpacas without crowding them! You don't want only 1, because they're herd animals, so they get very lonely and depressed when they're by themselves. :( A lot of times, it even leads to premature deaths!
Also, you'd want to split your pasture into 2 parts, to rotate them (so they don't chew down the grass too much, and you have time to seed the pastures).
I'll message your our phone number. We help new ranches get started ALL the time!! We would love to hear from you and help answer any more questions you may have. :)