You should declare the variable "_i" as a global variable.

Code

var _i=0; //global var

function on_space_event()
{
	for(; _i < 15; _i += 1) {
		if(random(1) < 0.5)
		A();
		else
		B();
	}
}


and if you want just one function to work each time you press the key, you don't need to use "for" loop. Something like this works:

Code
var _i=0;

function on_space_event()
{
	_i+=1;
	if( _i>15)
	{
		return;
	}

	if(random(1) < 0.5)
	A();
	else
	B();
}
}