A lot of things come into factor.
Has the mare has had foals before?
Is the mare in good physical shape?
Does she have any medical problems?
Does the mare have any previous injuries that could result in complications from carrying the foal, or delivering it?
There are a lot of things to look at. Can you cover all the expenses of a vet coming out to check her. Can you financially support 2 horses once the foal is born? Can you cover the costs of the mare is she has any complications with delivery?
There are many books out on caring for a broodmare. I would suggest also reading up information on the web.