Of course, this depends on whether these are appetizer kebobs or for an entree.

For appetizers, 6 inch skewers, 3 pieces of meat, a pound boneless will make 10-12 skewers. You will soak wooden skewers before threading if used for grilling.

It is very helpful to cook the veg kebobs separately, as they tend to be either overdone or underdone when cooked with the meats. See this off site help:
www.recipe.com/collections/grilled-vegetable-kabobs/

If these are appetizers, another option is to serve platters of the assorted meats and veggies grilled, and let people spear their own. There are 4" skewers available.

Also, be aware that given beef and chicken, almost 2/3 of most crowds will choose beef if they take only one, or one of each if they take 2.