Generally installs will require 2-3 bundles but it does depend on the length of your bundles. With shorter length bundles you won’t need as many because the wefts are fuller. But with longer-length bundles (22”+) you’ll use around 3-4 bundles because the wefts aren’t as full but you want your install to be full. 

If your install is going to have a frontal the bundles will stay in the 2-3 range, even the longer length bundles because the frontal is going to cover a good portion of your head. 

If your install is going to have a closure the bundles you’ll need will be anywhere from 2-3 up to 4 again depending on their length. The rules apply for a traditional install, 2-3 bundles  and more with longer length bundles. 

The exception to this rule is the texture of the hair. If you are installing curl hair you can get away with adding more bundles no matter the length because it just makes the hair look fuller. But with straight hair the more bundles you add it your hair won’t lay flat and will look stuffed almost.