Flow restrictors are sized in milliliters per minute. You would use a single flow restrictor of 750 to 1000 ml/m. I would base it partly on the TDS of the source water. If the water is already fairly soft, you could get by with 750 ml/m but with harder water (>250ppm) the membrane will last longer with a 1000 ml/m restrictor.
When you force a better waste-to-product ratio with a smaller flow restrictor you are shortening the life of the membrane, so its a balancing act between the cost of replacing membranes and the cost of your water.
http://www.purewaterproducts.com/art...ow-restrictors
Are you going to use a booster pump, or do you already have good pressure in the line?