The 20 amp circuits with little on them are probably there for a purpose. They may have been installed for the previous ower for specific appliances.If these circuits are in the kitchen, there should be no more than two outlets per circuit, and they are referred to as " appliance circuits" by code.
Not that you implied anything to make me think this, I caution you about adding any other outlets on the 20 amp circuits unless you know exactly what they are for ,of course, depending on where the circuits are located.
At the time your house was wired, dining rooms were required to have 20 amp circuits. This practice is no longer required by recent code editions.
Laundry areas are to have one 20 amp circuit, along with bathrooms, and of course kitchens. If the 20 amp circuits you have are not in these areas, they may have been for certain appliances the former occupant had, refrigerators, freezer, window air conditioners. If none of these examples apply, then you can add to the circuit, and take advantage of them.
As good rule of thumb is to have no more than 10 outlets on a 20 amp circuit. However, this rule does not apply to residential installations, only to all other installations other than residential, commercial such as stores, offices, etc, and to industriial such as factories, ware houses, etc.
Even thou does not apply to homes, I do agree that no more than 10 outlets on a 20 amp circuit is a good rule.
I enjoy arguing with people that think it must apply to homes, since the code does not require this, I try to be clear and informative. Funny how people make stuff up.
For example, code does not REQUIRE refrigerators to be on their own separate circuit. Having a reefer on its own circuit makes tremenous common sense, every home I have wired has the reefer on its own circuit.
The code lists requirements for minimums and maximums. Wiring better than the code requires is recommended, makes common sense, allows for peace of mind, and is done by every good electrician. But when someone says only a couple of outlets are needed in a certain are, the code will dictate if this is allowed. Used and understood properly the code can be your friend.
I realize you did not ask for this info, just thought I would respond to help with clarification and information to you and others who could benefit from it.
Thank you for listening.