we currently use PO-Subxxxx as PO number, but we want to auto generate lot number with only xxxx i can create a custom field, but not sure which formula to use to get only the number and not the letters.

substr({pofield}, 7); this works if your first 6 digits are always the same. if not, then you'd want to leverage regexp_substr() or maybe substr({pofield}, instr(unique char string to look for) + digits to add = unique char string length)