How To Handle It If People Don't RSVP
There are many reasons why receiving RSVPs for a party you are planning is important. For example, you may need definitive guest numbers so you can calculate the amount of food you need, or you may just want to know who exactly is planning to attend so you can meet any special requirements they may have. There is no doubt the people receiving your party invitations will know they should RSVP, but alas, some may simply forget. You're then left with a difficult situation of having someone arrive who you had not expected, and having to make room and allowances for their attendance.
If this does happen, try and keep your temper - even though it may be tempting to lash out or make a snide, sarcastic comment. The vast majority of people who fail to RSVP do not do it out of a deliberate desire to be impolite, but merely due to misplaced thinking; they forget, or think they already have, or mean to do it when they next see you. Always be gracious, otherwise you could ruin the atmosphere for the other guests.
As is the case in so many things, prevention is better than having to react to an event when it comes to RSVP. Firstly, make it clear on your invitation that you expect people to RSVP, but keep it friendly. For example: "For the sake of my sanity, please RSVP so I can get a firm grip on the numbers attending!".
If you are a few days before your party and still have had no RSVPs, it's worth getting on the 'phone and asking people if they're coming. You have a right to know, and providing you keep your tone light and friendly, there shouldn't be a problem.