How to book many marines quickly

In the late game, when I have 5-7 raxes I often need to book 15-20 marines (lings, etc) at a time. To do so right now I'm pressing 'a' button 15-20 times... which take me solid 5-10 seconds....

Is there any way to do that quicker? I believe I can configure my Windows to do 'auto-repeat' with SMALL delay, but I will need to do that before starting playing starcraft and change settings back once I left.

Is there any better way?

You'll need to modify your autorepeat settings and then simply hold down the key for an appropriate duration. This is the only way I'm aware of, as StarCraft doesn't allow you to configure that setting in-game.

In Windows, these two settings are in Keyboard Properties --> Speed --> Repeat Delay/Refresh Rate.

Protoss players have gotten pretty good at moving the mouse at an appropriate speed that matches the refresh rate of the keypress so as to warp in units quickly.

Unless you use an external source, this is not possible within the game of StarCraft.

Protoss can't warp in 15 stalkers to a pylon with a push of a button, Zerg can't make 20 roaches from larva with a push of a button, and Terran can't queue up 10 marines with a push of a button.

If you want to do it the fastest way, this is what I do. I have all my barracks on hotkey 2. I hit 2. I hold in A. If it looks like I have AROUND 10 marines, I let it go. If I look over, I hit ESC a couple of times to get it to a close number.

I will pretty much always choose to have 1 or 2 off and save a couple of seconds than to get the exact number of units and waste valuable time.