New pool owners here. I have an 18,000 gallon salt water pool. What is the recommended rpm that I should be running my pool pump. I currently have it set for 2400.

Check out Trouble free pool. Lots of good advice on that type of thing and chemicals.

I run mine a bit lower than that - 2100-2200 in the summer and 1700 in the winter for much briefer periods.. Technically, i think there is some water turnover goal you are shooting at, but I haven't really researched it. Long as the pool seems to be staying clean and clear, I think you're good.

What size pump? I have a 2.7hp e pump set up on a 26k pool running at 2850 rpm.

You'd probably be good at that speed or maybe 2500 for 8 hours a day

Get a flow meter to calculate gallons per minute at that speed. You want to turn over the pool at least once a day. Ideally you want the lowest flow to trigger your salt cell and for the least amount of time without causing algae. Anything more and you're wasting energy. 1 hour at a high speed to skim the water and the rest at a low speed to turn over the pool.