mbed-os/features/netsocket/emac-drivers/TARGET_STM_EMAC/mbed_lib.json

{
    "name": "stm32-emac",
    "config": {
        "eth-rxbufnb": 4,
        "eth-txbufnb": 4
    },
    "target_overrides": {
        "NUCLEO_F207ZG": {
            "eth-rxbufnb": 2,
            "eth-txbufnb": 4
        }
    }
}