How long does a cow stay in...

The gestation period for beef cows is 279 to 290 days and for dairy cows 262 to 300 days. In response to the second part of the question, it is not uncommon for cows to produce twins. We featured a story some years ago about a Hereford that produced quadruplets! So yes, a cow can support (we assume you mean “nurse”) more than one calf.