Determining the amount of wine you need for a wine tasting depends upon your wine, size of the group, and your intentions.
What will be the atmosphere of your wine tasting? Is this a serious tasting, where you will only try small amounts of several wines? Is it a casual wine tasting party, where guests are expected to "go back for more."
You should buy one full-size (750 ml) bottle for each six to eight guests. This will allow each person to try a small taste of each wine (about 2 ounces for the first taste). There will be a little left over for another taste, when the guests have determined their favorite wines.
Plan to serve five-six different wines at your tasting. Try not to have more than eight different types of wine, unless you want to try every Chardonnay available. Too many wines can be confusing.
If you know that a particular wine will be extremely popular, you may want to pick a couple extra bottles of that wine. This is hard to predict. I recently presented an Italian themed wine class. 
I included a "cheap" bottle of white table wine. I wanted to give the group an idea of budget Italian wine. After the excellent Chianti, the Fontana di Papa budget wine was the hit of the evening. During my next trip to Rome I intend to bring back a case of my $1 wonder.
This is a good lesson for selecting wines. Don't select wines based upon price. There are excellent wines available that are not expensive. This could be a theme for a wine tasting party. No wines over $10! Give a prize for the person who brings the best tasting wine with the lowest price tag.
Enjoy!
