How much cash reserve should you have in account for buying resale HDB?

Look at the average selling price of the type of units you are interested in.

Calculate your budget, you can do a IPA to estimate how much the bank will lend you.

See if your budget and cash can meet the average selling price. Best if can keep way within budget.

Look at the asking prices on the property listings and do your research. Probably can tell the difference between asking and selling. Have at least that as cash. Have more if you want to get hot units like near mrt or facilities.

If really not enough cash and cannot beg, borrow, steal, walk away and forfeit the option fee.