The rule of thumb with guests is 75% of invitees but you can't count on that because every situation is different. Here are a few things to consider...
The Size- Generally the more people you invite the higher the regrets percentage will be. This is not always the case though and can depend on your culture.
The Date- The date can be a big decision factor for guests. The middle of summer guests may have holiday plans, the middle of the week brings the issue of work commitments.
The Location- Is your venue in the middle of nowhere or on a holiday weekend? Guests may be less likely to travel far if they have to organise accommodation or travel on busy roads. If it is a destination wedding it is especially hard to know, sometimes it's too costly for guests and others make it into a holiday and you'll get more acceptances than you expected.
Guests Location- You may have less acceptances if your guests live overseas or have to travel quite far to your wedding.
When you send- If you send out your invitations well in advance you will have more acceptances than if you send them out last minute.
- Invite more people than your venue can accommodate, you never know they may all accept your invitation.
- Inviting guests in two lots, A & B lists. When you know how many on the A list cannot make it, you send out the B list. Ah but if the A's & B's should discuss when they got their invitation? Look out!
- For guests to accept and not show up
- To get unexpected guests, eg uninvited partners
- To call everyone on your guest list when they don't rsvp on time