You will find that the larger the party, the less games that will be played.
My last Wii party this last Thursday involved a group of about 15 people and there was no getting off Wii sports, as to give everyone a chance to play took even more time. While Tennis is fast paced, the Bowling does take a bit longer and only allows four at a time to play. If you are having guests like myself to whom have never played the Wii, Wii Sports is your best bet to get them started.
when you have more parties, or have guests that are more a-tuned to the Wii, you can more easily roll the title selection. Consider game specific themed parties depending the size of the group and play experience, however simply having one title is enough to keep a small crowd occupied for an evening, and keep a couple others ready on stand-by if by chance the mood begins to sour.
Also, MAKE SURE you have enough room to hold the crowd. My last party was to include the Wii, a cookout, and an evening camp fire marsh-mellow event. My wife and I decided to experiment with setting up the Wii in the yard to keep everyone concentrated in one area, provide more room, and keep everyone together. Mind you, the Wii and the sun do NOT get along.