  Knowledge of the number density of H$\alpha$ emitting galaxies is vital for
assessing the scientific impact of the Euclid and WFIRST missions. In this work
we present predictions from a galaxy formation model, Galacticus, for the
cumulative number counts of H$\alpha$-emitting galaxies. We couple Galacticus
to three different dust attenuation methods and examine the counts using each
method. A $\chi^2$ minimisation approach is used to compare the model
predictions to observed galaxy counts and calibrate the dust parameters. We
find that weak dust attenuation is required for the Galacticus counts to be
broadly consistent with the observations, though the optimum dust parameters
return large values for $\chi^2$, suggesting that further calibration of
Galacticus is necessary. The model predictions are also consistent with
observed estimates for the optical depth and the H$\alpha$ luminosity function.
Finally we present forecasts for the redshift distributions and number counts
for two Euclid-like and one WFIRST-like survey. For a Euclid-like survey with
redshift range $0.9\leqslant z\leqslant 1.8$ and H$\alpha+{\rm [NII]}$ blended
flux limit of $2\times 10^{-16}{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}$ we
predict a number density between 3900--4800 galaxies per square degree. For a
WFIRST-like survey with redshift range $1\leqslant z\leqslant 2$ and blended
flux limit of $1\times 10^{-16}{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}$ we
predict a number density between 10400--15200 galaxies per square degree.
