I don't think this is very true. I can't imagine getting an offer without knowing the total number of shares.
Part of the reason is that the "cap table" is usually a mish-mash of disorganized Excel spreadsheets after several rounds.
jwz had to battle Netscape HR to see those numbers.
But Delaware-registered companies must on request.
Knowing the percentage isn't all the meaningful since there will be dilution later, or more classes, liquidation preferences, etc.
#1 Be wary of late stage private companies offering large ISO grants, these grants are absolutely worthless as long as the valuation does not exceed FMV (fair market valuation) at time of grant.
#2 (ISOs) Start date FMV is not grant date FMV, I got burned by taking an offer at a startup where the implied valuation of the ISO options shot up dramatically after my start date, 400x by the time the board met and issued options after my start date(the companies valuation only went down from this peak).
#3 (ISOs) Make sure that the exercise window is a few years after you leave the company, given alot of large unicorns take 7-10 years to go public you don't want to get into a situation where you join a company, its valuation shoots up, and are either let go or take another job and are forced to exercise 100's of thousands of ISOs(in 30 to 90 days) leading to a mammoth tax bill which gains may not be realized(think weWork and its dramatic collapse) and you are still on the hook for this come tax time.