The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [OIII] and H$beta$ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at $zsim8$ and study their rest-frame optical emission lines. Combining data from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and from HST, we perform spectral energy distribution (SED) fitting, using synthetic SEDs from a large grid of photoionization models. The deep Spitzer/IRAC data combined with our models exploring a large parameter space enables to constrain the [OIII]+H$beta$ fluxes and equivalent widths for our sample, as well as the average physical properties of $zsim8$ galaxies, such as the ionizing photon production efficiency with $log(xi_mathrm{ion}/mathrm{erg}^{-1}hspace{1mm}mathrm{Hz})geq25.77$. We find a relatively tight correlation between the [OIII]+H$beta$ and UV luminosity, which we use to derive for the first time the [OIII]+H$beta$ luminosity function (LF) at $zsim8$. The $zsim8$ [OIII]+H$beta$ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [OIII]+H$beta$ luminosity at a given UV luminosity from $zsim3$ to $zsim8$. Finally, using the [OIII]+H$beta$ LF, we make predictions for JWST/NIRSpec number counts of $zsim8$ galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for $zsim8$ spectroscopy even at 1hr depth and JWST pre-imaging to $gtrsim30$ mag will be required.
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