If you want to link 25 uC to a PC, it is the best to use RS485 - 2 wire link. For safety reasons - optic fibre link would be the best solution, at least PC - uC network should be separated by fiber optics.

Each uC should have assigned unique network address. The software layer of the protocol is not so hard to do. There are also many existing protocols that are based on RS485 (modbus, profibus, tosline, canbus etc.) If protocol is open, you can implement one or make your own.

You send a command from the master (PC) to a slave address. All slaves receive it, but only addressed one replies. PC waits for a reply, if timeout, uC with given address is not operational or not existing. For error correction and detection you should buildin a CRC checksum calculation (8 or 16 bit). In general 16 bit CRC is better to use. The data messages - packets are in general small for you write or read only the small amount of data at the time (get a measure, write parameter to uC, etc...)