The Lions can add eight players to their practice squad, and so far five have been signed (that we know about):

WR Tim Toone
WR Mike Moore
DT Robert Callaway
LB Caleb Campbell
CB Paul Pratt

All five of those players were released by the Lions yesterday. I would imagine the final three spots, assuming the Lions use them, will include players they released yesterday as well, although free agents that weren't on the team and weren't picked up off waivers can also be signed to the practice squad.

An official announcement about the practice squad will likely be made in the next 24 hours.