Hey Yazeed
On your first question you are right, with this license you are able to manage up to 10 Firepower devices. On the second one you are right too!, if in the future you will add more device/sensors you will need to purchase another FMC license, but this other devices/sensors will be manage in a separate FMCv.
So if have different managements points is not a problem, you should be good, but if it is a problem you should go with an appliance that support more devices/sensors.
Juan