Ask HN: Job offer only lists shares that I'd receive, how much are they worth?

You'll want to know percentage of company, and if they've got a valuation of the company already what that value is. Unlikely if they're "fairly early" that they know their valuation, so ask them for their projected valuation at the key stages. If they're offering equity, that's lovely. But that means that they have to pull back the curtain a little bit so you know what it all means.

The value of being payed in shares by an early startup is calculated by dividing the number of shares by itself, and subtracting 1.

Ask how many shares have been authorized. Thats determined bybthe board of directors.