The limit of MODBUS slaves is defined by the standard itself and is 247. There is no other limit, regarding Teltonika‘s RutOS. This number can, however, be limited by the communications link. For example, RS485 has a limit of 31 slaves.
In terms of performance, no tests have been carried out. The end result is a sum of several factors: number of slaves, frequency at which requests are sent to the slave devices, available processing power, additional running services on the router, etc. Thus it is not possible to provide any estimates and the initial setup need to be tested.