No matter how big a drinker your best man is (!!) you will only need 1 or 2 kegs (at the most) for any wedding that is expecting in the region of 120-150 guests Make sure your bar provider has packaged options as back up and you will have plenty. There are about 88 pints in a standard 11 gallon keg.

No more than 6, and I usually recommend 4. This is because of the pre-prep required, the effect too much choice can have on the quality of service and the number of ingredients you will need to pay for.