Wow. That's a hard number to even estimate. Every time a blood cell in the lungs takes on an oxygen molecule, that's a chemical reaction. Every time a cell in the pancreas produces an insulin molecule, that is many chemical reactions. Every time a nerve cell in your eye or brain fires, this is a cascade of ions traveling through the nerve cell, which is essentially thousands of chemical reactions, which cascades out to thousands of other nerve cells.
But to give an idea of how big the number is, just to stay alive, a human cell has to turn over (consume and replace) all its ATP (energy source) about once every one or two minutes. So that's about 10^7 chemical reactions (a 1 with seven zeros) per second. (See source.)
Multiply that by 86,400 (8.64 x 10^5) seconds per day. That's 8.64 x 10^12 reactions per cell per day.
Multiply that by 100 to 200 trillion cells in the human body (depending on how big you are) ... so lets call that 100-trillion = 10^14.
So that's about 8.64 x 10^26 chemical reactions per day.
So next time someone asks "are you busy?" ... you can say "heck yes!"
Caveat: I did the above rather quickly, so check my math. I estimate that I consumed about 3.75 x 10^11 molecules of ATP in the act of doing that computation and typing into YA!.