adding watts to a circuit
You have a 30A 240V breaker (pair tied together) so this is a 240V circuit. You have 2X 500W on it now, or a load of 4.2 amps. You can add 9 more 500w heaters without violating the derated (80% capacity) load for heating circuits (which are considered as continuous use, i.e. more than 3 hours likely)

You can run a maximum of 5760 watts on this circuit (as derated properly.) In 500 Watt chunks that only gets you 11 - if you have some other combination that adds up to 5760 or less, you can run that combination.

This does assume that the wire is correctly sized for the breaker - such as 10 AWG copper or larger (size, smaller gauge number.)