SmileBASIC can't save a single number, but rather only arrays. To save this number, you have a couple of options.
You can save with SAVE "TXT:TEST",STR$(SUB) and load it with LOAD "TXT:TEST",FALSE OUT S$:SUB=VAL(S$)
You can save with DIM ARR[1]:ARR[0]=SUB:SAVE "DAT:TEST",ARR and load with DIM ARR[1]:LOAD "DAT:TEST",FALSE OUT ARR:SUB=ARR[0]
If you choose the second option, however, you'll have to make sure you only use DIM ARR[1] once, preferrably at the start of your program. If you DIM an array more than once, SmileBASIC gives an error.