You are allotted a certain number of CPU cores, and you can run as many virtual devices as that number of CPUs will permit at a time.
These CPU cores correspond to the number of CPU cores available on the Arm servers assigned to that account. Each virtual device requires a certain number of CPU cores in order to run.
For instance, if you have a 12-core account, you can spin up two 6-core devices for your first test run, then you could turn those off for storage and create six 2-core devices for the next test. For every two active CPU cores allotted to your account, you can store up to five devices in an Off state.
The number of cores per device:
Raspberry Pi 4 - Takes 4 cores
iMX8m Arm Cortex Complex - Takes 5 cores
STM32U5 IoT Discovery Kit - Takes 1 core