There are cleaner ways but a simple way is this
.timer 1 1 join #channel1
.timer 1 2 join #channel2
.timer 1 3 join #channel3
.timer 1 4 join #channel4
.timer 1 5 join #channel5
.timer 1 6 join #channel6
.timer 1 7 join #channel7
.timer 1 8 join #channel8
.timer 1 9 join #channel9
.timer 1 10 join #channel10
.timer 1 11 join #channel11
replace #channelN with your channel names of course
this sets of a 1 time only timer (the first 1), that goes of in N seconds (the second number), and does the command following it (the join #channel)
if joining 1 channel a second is still to fast, then just increase the delays
.timer 1 2 join #channel1
.timer 1 4 join #channel2
.timer 1 6 join #channel3
etc
