I know there are more elegent ways of doing this but as a starting point to learn I would suggest this:
Each byte received needs to have somewhere to go like the first character you did so make some more variables for the rest.
Code:
X0 VAR BYTE
....
X7 VAR BYTE

setnum:
SERIN rcx,n2400,X0,X1,X2,X3,X4,X5,X6,X7
WRITE 2, X0   ' Remember this takes some time, about 10ms ea
WRITE 3, X1
...
WRITE 9, X7
GOTO main

RetNum:
READ 2, X0
READ 3, X1
...
READ 9, X7
SEROUT tcx,n2400,[x0,X1,X2,X3,X4,X5,X6,X7]
goto main.
I haven't tested this but I hope you get the idea.