Are alpacas related to llamas?

Alpacas are very closely related to llamas. They are both from a group of four species known as South American Camelids. The llama is approximately twice the size of an alpaca with banana shaped ears and is principally used as a pack animal. Alpacas are primarily bred as fleece animals in Australia.

How many animals can I run on my property?

That will depend on what sort of pasture and how much pasture your land is capable of producing. Different climatic regions and different soil types vary widely in their carrying capacity.
A standard unit of carry capacity is the Dry Sheep Equivalent per hectare (DSE). For example, in areas of good soil and high rainfall your property might sustain 10 DSE/ha, compared with dryland areas that might be 1.5 DSE/ha.
The DSE for your property can be determined by speaking to an agricultural consultant, or perhaps your neighbour if they are experienced farmers.
As a general rule, one alpaca wether is equivalent to one DSE. If you are perpared to supplementary feed with extra hay, you may be able to increase your stocking rate. A pregnant alpaca is approximately 1.5 DSE and a lactating pregnant alpacas is about 2 DSE.