Little Man Computer Help Please!!!

My program's purpose is to allow user inputs until zero is entered or there is no space for it to be stored. I need to store each number in another mailbox and count the numbers I have stored. Here's my program.

00 INP 901
01 STOR 397
02 LDA 598
03 ADD 199
04 STOR 398
05 BRZ 707
06 BR 600
07 LDA 598
08 OUT 902
09 HLT 000
97 DAT 000
98 DAT 000
99 DAT 001

I am having trouble figuring out how I can make my program store the number in different mailboxes and how to make the program stop when there is no more space. Thank you in advance!