Wine Tasting ...

Horizontal tastings and Vertical tastings of wine are two different ways of comparing wines side by side. Since the best way to judge the differences, or for that matter the similarities in wines is to taste them side by side, some different concepts have developed regarding the organization of those tastings.
When you are going to hold a wine tasting, you can decide to have: a general tasting, a vertical tasting or a horizontal tasting. The differences between the types are:

General Tasting ...  This is a free-for-all. There are no real limitations on the wines that you have involved. You may, of course, limit the wines to those from a particular part of the world, or grape variety, or price range or whatever other criteria that you select.
Horizontal Tasting ... A Horizontal Tasting involves wines that all come from the same vintage. You decide the vintage and you determine if you are going to place any other limitations on the wines involved. You might decide to have only red wines, or wines from a single grape variety or just "Bordeaux First Growths"; but, the wines will all be from a single vintage. A horizontal tasting is a good way to see which winery was most successful within a given year. You can also begin to detect styles for which a given winery may be known. For instance, if you had a horizontal tasting of wines that all came from the 1997 vintage and were only Cabernet Sauvignons from the Claire Valley in Australia, you would find that some of the wines were 'bigger' and had more aging potential. Others might express more forward bouquet at an early age. This type of tasting tells you more about the producers than about the vintage since you are really comparing numerous wineries instead of different years.

Vertical Tasting ... A vertical tasting involves wines from different vintages but all the wines will come from the same winery. If the winery produces more than one type of wine, you would select a single wine from that winery and taste multiple vintages of that wine. For instance, you might have 5 vintages of Chateau Mouton Rothschild (a famous wine from Bordeaux in France). When you hold a Vertical tasting, you are learning more about the differences between different vintages rather than the differences in wineries.
There is no rule or accepted practice about the number of wines that you need for a Horizontal or Vertical Wine Tasting. You could just have two wines tasted together and that would qualify. As a guide, you would be best to have a minimum of three wines or, more usually, 5 or 6 wines to make a more interesting tasting.
Wine and Food Festivals may upwards of 40 different wines available, but this can be overwhelming, and your tastebuds quickly confused!