They may have reasons for not wanting to give exact numbers, because backing out changes in this number over time can give information on the compensation of yo

They may have reasons for not wanting to give exact numbers, because backing out changes in this number over time can give information on the compensation of your peers. However, any honest company should be willing to give you numbers in writing to within an error of, say, +/- 1-2%. Which should be enough precision to make your decision. Variations in potential future valuations far exceed this error, so the exact number shouldn't be terribly crucial. On the other hand, any company that gives you no information on the number of outstanding shares makes their options worthless as compensation. You should let them know this on your way out the door.