Have a close look at the syntax options available for the SerIn command - particularly with the "#" qualifier, which gives you the option to receive a string of digit characters "0", "1",...."9" (Eg 3 characters as the string "318", which is made up a 3 ASCII characters $33, $31, $38) and put them into a suitably sized register (a word in this case) as the decimal value 318 ($13E in hexadecimal)
Note that the M2 series of chips of PICAXEs could struggle to accept data arriving at baud rates faster than 4800 in many circumstances, unless each character is spaced slightly from the previous one to allow the PICAXE to process and save each character. You will have to experiment a little to see what the capabilities are with your particular situation.