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My guess is he's only using a couple of those rectos at a time, and most are backups.
Some of those amps are for the stage volume as well. According to the guys at Kemper, Till doesn't use in-ears, he prefers old-school stage volume.
It's funny though because they (Richard at least) recently moved to kempers, but still use like 5 of them. AND still have several amp heads on hand for the previously-mentioned stage monitoring. Someone did point out that he probably uses so many Kempers (and previously amps) at once because he runs a stereo rig. One preset is labeled "Zeig Dich L" and some are "Zeig Dich R", Zeig Dich being a song on their setlist and the L/R obviously meaning left and right.