  Many extrasolar planetary systems containing multiple super-Earths have been
discovered. N-body simulations taking into account standard type-I planetary
migration suggest that protoplanets are captured into mean-motion resonant
orbits near the inner disk edge at which the migration is halted. Previous
N-body simulations suggested that orbital stability of the resonant systems
depends on number of the captured planets. In the unstable case, through close
scattering and merging between planets, non-resonant multiple systems are
finally formed. In this paper, we investigate the critical number of the
resonantly trapped planets beyond which orbital instability occurs after disk
gas depletion. We find that when the total number of planets ($N$) is larger
than the critical number ($N_{\rm crit}$), crossing time that is a timescale of
initiation of the orbital instability is similar to non-resonant cases, while
the orbital instability never occurs within the orbital calculation time
($10^8$ Kepler time) for $N\leq N_{\rm crit}$. Thus, the transition of crossing
time across the critical number is drastic. When all the planets are trapped in
7:6 resonance of adjacent pairs, $N_{\rm crit} = 4$. We examine the dependence
of the critical number of 4:3, 6:5 and 8:7 resonance by changing the orbital
separation in mutual Hill radii and planetary mass. The critical number
increases with increasing the orbital separation in mutual Hill radii with
fixed planetary mass and increases with increasing planetary mass with fixed
the orbital separation in mutual Hill radii. We also calculate the case of a
system which is not composed of the same resonance. The sharp transition of the
stability can be responsible for the diversity of multiple super-Earths
(non-resonant or resonant), that is being revealed by $Kepler$ mission.
