Why are both lithium and cesium alkali metals, when lithium has three electrons and cesium has 55

Because they both have one electron in their outermost shell

Explanation:

Both lithium and cesium are classified as alkali metals because they both have just one electron in their outermost shell.

 The electronic configuration of both atoms are:

          Lithium  = 2, 1

          Cesium  = 2, 8, 18, 18, 8, 1

Now we can see that the outer shell of both elements have just one electrons.

Division into groups is based on the number of outer most shell of each element

it can be icreased by increasing pressure.