Well, I figure we can run a minimum of 5 ewes per cow/calf pair.  Now, we don't let our lambs out on pasture with the ewes.  The lambs get weaned at 7-9 weeks.  Rotational grazing helps.

 Can't tell you how much they drink, but the one concern I would have on a water tank is it might be too high for them to reach, unless it's low tank.  We have fountains that are lower then the standard cattle fountain and put a half rail road tie in front for the lambs to step on to drink.

Gestation is 142-148 days, depending on breed and  how inconvenient it is for you get lambs at that particular moment.  They can come in a few days earlier too.

I would recommend feeding oxytetracycline or chloretetracyline starting 5-6 weeks ahead of lambing to prevent Clamidia (aborts).  Nothing more frustrating then to have a lamb be born 3 weeks early.