How many air changes an hour does this unit handle
The air changes per hour depends on the CFM and the size of the room. This machine with 1250 CFM in a room size 25'x28'x9' can do 12 Air changes per hour. If you give us your room size we can calculate the air changes for you.
I have a 40foot x 60foot x 9 foot ceilings will two of these be enough?
Yes, 2 of these machines will get you good results.