why use "pourra" instead of "pourront"?

Question

En cas de guerre, tout citoyen ________ être appelé à participer à la défense du pays.why use "pourra" instead of "pourront"? I thought tout means every, all.

Cécile Janty-Davies · Accepted Answer

Bonjour Chaniya, 
The French phrase "tout citoyen"  translates to "every citizen" or "any citizen" in English.
 
The phrase is commonly used in legal or political contexts, such as in declarations of rights or constitutional texts, to emphasize that a particular right or obligation applies universally to all members of the citizenry.
 
As citoyen is masculine and singular, 'tout' is used and the correct verb form will be the 3rd person singular ---> pourra 
Bonne Continuation !