The number of trains is going to depend upon the starting feed water quality and the resulting water quality you want. There are many equipment manufacturers that will provide a system for the Middle East. Do a search of a wide variety of Water Treatment Equipment web sites buyers guides or use your favorite search engine for "DEMINERALIZERS".
I recommend three identical trains. Size each train to handle 50 percent of the total load. Operate two trains in parallel while the third train is either in standby or regenerating mode. Whenever a unit needs to regenerate, then the train in the standby position comes on line to take it's place and the exhausted unit regenerates and goes into standby mode...and so on. The third train also allows you to take another train down for routine service and maintenance.
Without a complete water analysis, it is hard to determine the volume of ion exchange resins required. However, you can estimate the size of the vessels based on the resin manufacturer's recommended hydraulic loading parameters. As a general rule service flow rate is typically 3 gallons per minute per cubic foot of resin and the typical resin bed depth is 36 - 48 inches. This rule applies only to makeup water only.
For condensate polishing, the hydraulic loading rate is typically 25 to 30 gallons per minute per square foot of plan area and the bed depth is 24 - 36 inches.
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You can do it with 3 trains if you don't object to 3.6 meter and larger diameter service vessels and you use a packed bed design. At that you will be near maximum hydraulic loading velocities. Without the benefit of a feed water analysis and a specification for product water purity, going with 3 trains could put you on the edge. My personal preference is to use vessels no larger than 3 meter in diameter.
How hot is your condensate? If you have a typical refinery condensate at 80-90 C you can't use a mixed bed desing without cooling it below 48.9 C.
One parameter that needs to be considered is the operating pressure of the steam generators and how much boiler blowdown you can tolerate.