How can I guess/predict how many guests will RSVP 'Yes'?!

My question is why do you need to know this before actual RSVP date?  

If you haven't already sent your invitations and you really need to know the answer then I'd put an earlier RSVP date.

Or just guess that there will be people who can't possibly go due to illness or circumstance.  

Don't go to great expense before you know the answer.  

Hi there!

Have you sent any Save the Dates yet?

The best way to gauge whether some guests can't come is to have a line on your Save the Dates that say something along the lines of "If you think you can't make it, please contact us on best contact number / email"

Otherwise the best thing to do is have a very early RSVP date on your invitations, which will allow you to invite more people down the track if you receive more "No's" than expected.

Please let me know if you have any more questions.

Cheers,

Lava @LavaStationery