The number of alkali metal(s) capable of forming superoxide amongst (Li, Na, K) is:

Lithium and sodium are capable of forming superoxide.

The inorganic substance with the formula ${\mathrm{NaO}}_2$ is sodium superoxide. The superoxide anion salt is represented by this yellow-orange solid. It is a step in the process where oxygen oxidises sodium.
Burning molten potassium in an environment with too much oxygen results in the production of potassium superoxide.

Hence, the number of alkali metal(s) capable of forming superoxide amongst (Li, Na, K) is 2.