Q.We know that the relationship between Kc and Kp is
Kp = Kc (RT)^Δn
What would be the value of Δn for the reaction
NH4Cl (s) ⇌ NH3

$K_P$ and $K_C$ describe the same equilibrium, so a fixed relationship connects them. Starting from the ideal gas equation $P=(n/V)RT$ and noting that active mass equals molar concentration $n/V$, each species' partial pressure can be written as (concentration)$\times RT$; substituting this into the $K_P$ expression and comparing with $K_C$ gives

where $\Delta n_g$ is the difference between the total moles of gaseous products and the total moles of gaseous reactants in the balanced equation. Three regimes follow: $\Delta n_g=0$ gives $K_P=K_C$ exactly (e.g. $H_2+I_2\rightleftharpoons 2HI$, or ammonia synthesis's own $K_C/K_P=(RT)^2$ inverse case where $\Delta n_g=-2$ gives $K_P=K_C(RT)^{-2}$, so $K_C/K_P=(RT)^2$); $\Delta n_g>0$ gives $K_P>K_C$ (e.g. $PCl_5\rightleftharpoons PCl_3+Cl_2$, where $\Delta n_g=+1$, so $K_C/K_P=1/(RT)$); and $\Delta n_g<0$ gives $K_P<K_C$ (e.g. $2SO_2+O_2\rightleftharpoons 2SO_3$).

This relation is the standard tool for converting a measured $K_P$ into $K_C$ or vice versa -- for example, $SrCO_3(s)\rightleftharpoons SrO(s)+CO_2(g)$ has $\Delta n_g=+1$ (only $CO_2$ is gaseous), so $K_C=K_P/(RT)$; at $K_P=2.2\times10^{-4}$ atm and $T=1002$ K, $K_C=\dfrac{2.2\times10^{-4}}{0.0821\times1002}\approx2.67\times10^{-6}$ mol L$^{-1}$. It equally tells us, purely from the sign of $\Delta n_g$, whether $K_C$ must be larger or smaller than $K_P$ for a given reaction without any numbers at all -- if a gaseous homogeneous reaction has more moles of product than reactant ($\Delta n_g>0$), then since $K_P=K_C(RT)^{\Delta n_g}$ with $RT>1$ in most common units, $K_P>K_C$, i.e. $K_C$ is the smaller of the two.