  The 21-cm forest is a sensitive probe for the early heating process and
small-scale structures during the epoch of reionization (EoR), to be realized
with the upcoming Square Kilometre Array (SKA). Its detection relies on the
availability of radio-bright background sources, among which the radio-loud
quasars are very promising, but their abundance during the EoR is still poorly
constrained due to limited observations. Here, we use a physics-driven model to
forecast future radio-loud quasar observations. We fit the parameters of the
model using observational data of high-redshift quasars. Assuming Eddington
accretion, the model yields an average lifetime of $t_{\rm q} \sim 10^{5.3}$yr
for quasars at $z\sim6$, consistent with recent results obtained from quasar
proximity zone pre-study. We show that if the radio-loud fraction of quasars
evolves with redshift, it will significantly reduce the abundance of observable
radio-loud quasars in the SKA era, making 21-cm forest studies challenging.
With a constant radio-loud fraction, our model suggests that a one-year sky
survey conducted with SKA-LOW has the capability to detect approximately 20
radio-loud quasars at $z\sim 9$, with sufficient sensitivity to resolve
individual 21-cm forest lines.
