The Grandstream HT502 can handle up to 3 RENs worth of devices connected to it's phone ports.
FYI: REN or Ringer Equivalence Number represents the electrical loading effect that a telephone ringer has on a telephone line. If a line is loaded with more devices than it is able to support, it may begin to function erratically or may cease functioning altogether.
The REN of each of your handsets should be listed on a sticker attached to the phone handset itself.
To calculate the full REN load of all of your devices you will need to add together each of their individual REN values. If this results in a total value which is less than 3, you're good to go!
Older phones generally have a REN of 1 while newer phones range between 0.1 for units which receive power from an outlet, to 0.8 for phones which are powered exclusively through the telephone wire.