I guess the ones we see at every race are the series sponsors (Pirelli, DHL, etc). They’re partners of F1 in some way or another. Near enough every race series has a title sponsor or official partners (BTCC has Dunlop/HiQ, Indycar has Izod).
Then there’s th title sponsor of any given race, usually along the pit wall, behind the podium, and around the track.
So yeah, there will only be 4 or 5 main sponsors at every race, apart from Monaco, which is still allowed to sell its advertising space to whoever it wants and receive all the money from it.