The pump size depends on the configuration of the membranes. I assume you are using 2 1/2 inch membranes? This set-up will probably work fine if you split the flow between two banks of two membranes in series. There are three flow parameters to worry about to get optimum performance and membrane life. The maximum flow through a 2 1/2 inch membrane is 6 gpm to prevent mechanical damage to the membrane. The minimum recommended flow rate is 1 gpm on the outlet concentrate flow to minimize fouling. And then you need to keep the recovery close to 15%. Your system will work outside these parameters but it may not perform as well. With this configuration and 100 gph pump you would have 50 gph of flow through each side. If you run once through without concentrate recirculation you may have to reduce the pressure to keep the permeation rate down so that the concentrate flow doesn't drop too low coming out of the second membrane. There is no absolute "right" way to set it up but some ways work better than others.
Leader 1/2 pint - Kawasaki Mule - Smoky Lake Filter Bottler
24 GPH RO, 2 1/2 x 40 NF3 (NF270), 140 GPH (Brass with no relief valve ) ProCon pump
2013 - 44 taps - 16 gallons syrup, 2014 - 109 taps - 26 gallons syrup
2015 - 71 taps - 13.5 gallons syrup, 2016 - 125 taps - 24.25 gallons syrup
2017 - 129 taps - 17.5 gallons syrup, 2018 - 128 taps- 18 gallons syrup
2019 -130 taps - 18.5 gallons syrup, 2020 ~125 taps-19.75 gallons syrup