I'll be going on a 5 days vacation in September. How do I make sure my stash is enough for my fully breastfed lo? He is mostly direct latching and I'm nursing on demand.

Like what Vanessa has mentioned, pump after latching to start building your stash. My friends did that when they were preparing to go back to work after their maternity leave ends.

After nursing, go pump it out. U need to train him bottle feed from now cos it take a while for baby to learn.