If you get an offer letter from a startup that contains a certain amount of stock options but the employer won't tell you how many outstanding shares there are, that's SUPER shady. Here's why: 
 
I've been at 8 startups and received stock options with all 8 of them. Stock options are meant to be part of your compensation package. 
 
In order to value the QUANTITY of stock options the employer is giving you, you need to know how many OUTSTANDING, FULLY DILUTED shares there are so you can roughly calculate the % of the company those shares represent. 
 
And when you know that % and you know what the company approximate value is, you can fairly evaluate your offer. 
 
This is especially important when comparing two offers, A & B 
 
A) One offer contains 10,000 stock options and 
B) one contains 100,000 stock options. 
 
Which one is the better offer? 
 
You can only compare this when you know what % of the company those shares represent and what the company is worth. 
 
Offer A 
10k shares 
100k outstanding shares 
10% "ownership" 
$10M company value 
$1m = stock options worth 
 
Offer B 
100k shares 
10M shares 
1% "ownership" 
$10M company value 
$100k - stock option worth 
 
All things considered, option A with less stock options is your better offer (assuming the stock options are even worth anything somewhere down the line). 
 
But nobody teaches you how to evaluate these offers, least of all the employer.