The maximum number of Modbus slave devices is specified in the Modbus standard itself. Modbus standard sets a limit on the maximum number of Modbus slave devices, which is 247. This means that Teltonika devices can support Modbus slaves up to 247, including 247.
It's possible that older firmware versions could have allowed for slave IDs just slightly over the limit of 247. However, improvements have been made to the Modbus functionality since then, and now, as a result, it conforms more closely to the Modbus standard.