Is there a limit of series connected LEDs when driving them with AC?
AC grid voltage may be 220VRMS but it's the peak voltage that dictates the peak current through the LEDs and the peak voltage of 220V AC is \$\sqrt2\$ higher at 311 volts.

The next issue with the formula is that when N is large, forward conduction won't happen until the instantaneous AC voltage rises above N*2.2 volts. This means that the peak of the LED current may indeed rise to 100mA but for a shorter period of time compared to when N is small.

This will mean that as N increases (and C increases in order to push a peak current of 100mA into the string of LEDs), the duration in time that the current is flowing gets smaller and smaller and the LEDs get dimmer and dimmer.

If not saying this method isn't OK but I am saying the formula doesn't really prove useful for anything at the moment.