How much does horizontal velocity change each second as it moves through the air?

Well, Bianca, I'll tell ya. You really haven't told us enough to completely define exactly what's going on here.  In fact, now that I look at it carefully, you've told us nothing at all ... only asked a question based on zero given information.

IF you're talking about an object like a baseball or a rock or another projectile of some kind, and IF the object has no jet engines on it, and IF it has been tossed, launched, thrown, or otherwise sent on its way to somewhere other than the place it left from, and IF it's OK to ignore the effects of air resistance, then ...

-- the only force acting on the object is the force of gravity

-- gravity only points down

-- gravity has no effect on horizontal velocity

-- horizontal velocity is constant, and doesn't change as time goes on.

But IF any of these things is different, then you can forget everything I said.  I'm just guessing.