Can I run a 21 pitch laser 2 prop on that motor its on a 17.6 triton boat.

What prop do you have on it now and what are the max rpms?

It has a power tech 19 pitch on it and it runs my elms to the alarm somebody told me I needed to put a 20 pitch on it

Be very careful not to lug an opti. MOST of the time, the manufacturer gets it right. I can set off the alarm on my 115 opti, then trim it out. Odds are you will not get a speed increase by going up in pitch unless you get a custom prop. I stepped up the pitch to a 21 on mine and lost 500 RPM and a bunch of hole shot, and didn't go any faster.

Check with your dealer and see if they'll let you test one. That's the best solution.