We've just started installing pressure treated 2x4s as sole plates in our 5 year old basement. We drove 3 nails into each 8' plate, one at each end (around 3 or 4 inches in) and one somewhere at the center.
For smaller lengths, we may use two, or in the case or plates next to doors that in turn abut walls, there probably won't be room for more than one.
I can't find a reference that specifies how many such fasteners are needed per unbroken length, or a recommended / required distance between such fasteners.
Are we doing right with this many fasteners to a plate?
If it helps, we're in East-Central Minnesota. And I'm using 22-caliber powder-actuated 2.5" nails, if that makes a difference to the recommendation.