How to calculate the cation exchange resin capacity? If I have a cation exchange membrane and need to remove 2 g/L of sodium, how much resin would I need. 

I used the amount of substance of $\ce{Na}$ to calculate the amount of substance of $\ce{SO3H-}$ needed for complete sodium removal. But this will not give me the correct mass since the resin is a polymer with that functional group attached. However I haven't been able to find any kind of complete structures for a cation resin.
First of all, you will need more sulfonic groups than sodium ions since sodium doesn't have a very good affinity for sulfonic groups.

Anyway, you can find some specs here — figure 2. This source is using sulfonated polystyrene with various crosslink percentages and expresses the capacity in $\mathrm{mmol}_\text{sulfonic groups}/\mathrm{g}_\text{resin}$ ($1\ \mathrm{meq} = 1\ \mathrm{mmol}$).