Can you post your code?
Serial.print(123, DEC) sends three bytes of serial data: the characters '1', '2', '3' (e.g. 49, 50, 51). To receive the value on the other board, you'll need to collect these bytes and reassemble them into a value. E.g. each time you get a byte, you want to do something like:
int c = Serial.read();
int digit = c - '0'; // '0' == 48: the ASCII value of the digit 0
value = value * 10 + d;
You'll need to send some sort of terminating character (like a newline) and check for that on the receiving end.
Alternatively, you can just send binary bytes (remember a byte only goes from 0-255) or multiple bytes, but then you won't be able to read the data in the serial monitor.