The MacArthur HAT is probably on the extreme end in number of GPIOs used. So if your HAT works with that, it's probably going to play with most other HATs too.
The only additional pins to avoid are 27 (GPIO0) and 28 (GPIO1). These pins are reserved for an I2C EEPROM to store configuration info for the Pi.
As Holger wrote, there is no hard standard. But ignoring MacArthur, most HATs are connecting through UART0, SPI0, or I2C1. A trick to work around unexpected conflicts is to add a jumper on the trace connecting to the Pi GPIO, either as a solder jumper, or as a pin header like we use for the CAN termination. That way it's easy to rewire the connection if needed.
The only additional pins to avoid are 27 (GPIO0) and 28 (GPIO1). These pins are reserved for an I2C EEPROM to store configuration info for the Pi.
As Holger wrote, there is no hard standard. But ignoring MacArthur, most HATs are connecting through UART0, SPI0, or I2C1. A trick to work around unexpected conflicts is to add a jumper on the trace connecting to the Pi GPIO, either as a solder jumper, or as a pin header like we use for the CAN termination. That way it's easy to rewire the connection if needed.