How to calculate the cation exchange resin capacity?
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 $\mathrm{mmol}_\text{sulfonic groups}/\mathrm{g}_\text{resin}$ ($1\ \mathrm{meq} = 1\ \mathrm{mmol}$).