Number of RR Lyrae (N _RRL) as a function of M _V for the 94 galaxies listed in Table 4. To deal with the ”log of zero” problem on the y-axis (in log-scale), we added +1 to the N _RRL. The green solid curve represents the empirical relation derived in Martínez-Vázquez et al. (2019). The dashed curve are for the different selected values of specific RR Lyrae frequency S _RR. Several "outliers" are also marked in this plot. The inset figure is the zoomed-in version for UFD galaxies with N _RRL < 7, where Virgo III is marked as a magenta star. For clarity, error bars and the curves for S _RR < 1 are omitted in the inset figure. Note that the y-axis is in linear scale in the inset figure.