In general the rates quoted are based on the number of rooms agreed. When you sign the hotel agreement you are undertaking to pay for those rooms.
If you seek to reduce rooms post agreement but before the cancellation deadline the hotels may not allow this and may insist you cancel the entire booking.
The vast majority of hotels will not accept any changes that reduce the contract value when you are past your free cancellation period.
If you seek to increase rooms hotels are generally more helpful, but additional rooms may not be available (especially in busy cities) or may be more expensive than the rate you have already agreed. If the hotel does have available rooms they may be happy to offer you further rooms at the contracted rate but you should not rely on this as it is totaly at the hotels descretion.
A few hotels do provide a "flex" style contract which combine a minimum number of rooms with the option to add more (subject to availabilty) at a pre determined rate. These types of agreeement are relatively rare however, and in our experience not that useful because the "subject to availability" qualification means that at busy locations rooms are not available.