If you don't know (minimally) the fully diluted count of shares in the company, your option grant is as specific is saying that your salary will be denominated in dollars and you'll be given more detail about it sometime after joining the company, pinky-swear.

A company which won't provide enough information to value an option grant contemporaneous with the written offer is being either abusive or stupid. There exist employment opportunities at companies which are neither abusive nor stupid. Take one of those instead.