There are a few fields in ND. all of them are outdoor fields, so nothing going until its warms up. These are listed in relative geological order from Minot
By-pass paintball is near Minot. I don't know when it is open, I heard they only do reserved days (no open play) but I heard that 5th hand.
3 Brothers Paintball, near Bismarck ND, is open every other weekend (Saturday) and see 100+ people for those weekends. They have a woodsball and speedball field. The speedball is mostly player-run so it depends on whom is out there on that weekend. Many of the NDSU paintball team members live near bismarck and are out there every open play, as are a few Members of Fargo United
Goose River Paintball is in between Grand Forks and Fargo ND (its my home field) and is open every Sunday. They have a woodsball/urban field and a Speedball field. Fargo United and NDSU paintball practice out at GRP. Generally the speedball field is setup every other weekend with Fargo United's practice schedule. GRP sees about 30-50 a weekend.
Valley Paintball is located 20ish miles west of Fargo ND, near Casselton ND. They are open every Saturday and has one large field with elements built from spools, hyperball tubes and a castle built from totes. I am not sure what Valley sees for players, I believe it is regularly 50+.
If you need an indoor paintball fix, Air Assault Paintball in St. Paul Mn is your best option. If you play speedball, their advanced play on Sundays is a great time. During advanced play they do race-2 points, (first) case of paint is $60 which includes entry and all day air. That runs from 12-4p. They also do tournaments about once a month. Fargo United (the team I play for) took first and second this weekend at a D5 3-man there; NDSU paintball took 1st at a D4 4-man last month.