I'd say the majority of people on minoxidil take a hit to the density of their hair in the first two months of their treatment - the minoxidil shed. Hair should come back thicker from this. After that, the only issues you'll have will be the wetness of the minoxidil decreasing the volume of your hair at times, and round about 3/4 months from the shed, you may notice another, mini-shed, as a sizeable proportion of the minoxidil hairs from the first shed enter their resting phase.
If you cease minoxidil, all the hair you have regrown as a result of minoxidil will fall out and you will return to the state you would be at had you never used it.