Pool is 23-24000 gal and I am running pump 8 hours per day. Will the RC 35/22 be large enough or would I be better off with the going with the RC 42? The price points on the RC 35/22 are alot lower than the RC 42 SO I think it would be less expensive over x number of years but I am concerned it will produce enough chlorine. I live in Michigan and pool is open from about mid May to mid September.

With a relatively short run time of 8 hours, you will need to upsize the cell to be able to produce enough chlorine in the available time. I would suggest the RC-52 cell.

If I increased pump run time to 10 hrs per day would the 35/22 work?

You could probably use the RC-42 if you increased the run time to 12 hours.

If you really want the RC-35, then you could supplement the chlorine periodically with liquid chlorine or bleach, or increase the run time during periods of peak use and high chlorine demand.

Of course, a lot will depend on your chlorine demand. Do you know how many ppm of FC you use on a daily basis? High, low and average?