How many meats can make a meatball?
You should be able to combine as many as you want.  Pay attention to the fat content in each meat, as that can make a big difference in the yield as well as the texture.  However, even if you make a fairly significant change (for example, from 20% fat to 10% fat), it still won't be wrong or inedible, just different.  (Although, I wouldn't recommend trying anything insane like 95% fat.  It's no longer a meatball/meatloaf at that point.)

The meatballs I regularly make were derived from a Swedish meatball recipe.  I use 100% ground beef instead of the other meats, and it works really well.  Also, instead of the traditional Swedish meatball sauce, I use marinara sauce.  It's delicious!