The number of hydroxide ions produced by one molecule of Na₂CO₃ on hydrolysis 
 

On hydrolysis,

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{CO}}_3+2{\mathrm{H}}_2\mathrm{O} \leftrightharpoons 2\mathrm{NaOH}+{\mathrm{H}}_2{\mathrm{CO}}_3 \\ 2\mathrm{NaOH} \stackrel{ }{\to } 2{\mathrm{Na}}^{+} +2{\mathrm{OH}}^{-} \end{gathered}$$

We can see that it gives two moles of hydroxide ions.