How much more salt can I dissolve in hot water compared to cold water?
There seems to be two parts to your question. First, not all substances have increased solubility as water temperature rises. Gasses for example have decreased solubility in this case. 

As for the second part of the question, increased heat adds energy to the water molecules which makes it easier to overcome the bonds between $\ce{Na}$ and $\ce{Cl}$. According to Wikipedia, the solubility of $\ce{NaCl}$ in nearly freezing water is $35.65 \, \mathrm{g} / 100 \, \mathrm{mL} \, \ce{H2O}$ versus boiling water which is $38.99 \,\mathrm{g} / 100 \, \mathrm{mL}\, \ce{H2O}$. Hope this helped.