Ways to make player shoot one bullet at a time?

[monstersstuck1]

I am making a game and you use the mouse to move your character, and WASD to shoot.

W = shoots up
A = shoots left
S = shoots down
D = shoots right

The problem is you can shoot multiple bullets at different directions at the SAME time. I want it so that you are only able to shoot once at a time. Basically to prevent people from cheating.

I hope you guys can help me out. Thanks.

[monstersstuck1]

And maybe have the player wait a quarter of a second before they can fire another bullet again.

[1Ombra1]

You can check the numbers of bullets in play area and shoot only when equals to zero.
Be sure to destroy the first static bullet and the exiting launched bullets.