Teams have 16 spots to fill, with the possibility for a 17th if the player has an International Pathway Program exemption (outside linebacker Julius Welschof has filled this role recently for Pittsburgh).
Ten practice squad players must have no more than two accrued NFL seasons (accrued means on a roster, injured reserve, or PUP list for six games or more).
The other six can be veterans, with no limits on their past NFL experience.
Teams can temporarily promote two practice squad players per week to the active roster. However, they can only do so up to three times per player before having to officially sign that player to the 53-man roster.
Teams can “protect” four practice squad players each week from being poached (signed to another 53-man roster) by another team.