In our experience, they will stamp you out and stamp you back in each time you leave the country, so it will use a new carnet page each time.
You could try the laissez passez option. Coming into Senegal from Guinea Bissau/Guinea/Gambia, the Senegalese customs are much less strict and corrupt than those at Rosso, Mauritania. I don't know if they enforce the 8 years old vehicle rule strictly at these borders like they do at Rosso?
If they don't you could re-enter on a laissez passez to save you carnet pages.
Official price for the LP is 2,500CFA.