Unless you have control over the network settings, you won't be allowed to join that many channels on any networks I've seen. There is a limit on channels on each network. In addition, mIRC can only open a certain number of windows at a time due to limitations in Windows.
Beyond that, mIRC cannot process that many lines (5 million per second!) that quickly without severe lag.
If you want a bot that stays in that many channels, you should look into an eggdrop or some other similar method. It just won't work well in mIRC.
Also, a bot can still be flooded off regardless of being an OP. The network can kick it for flooding even if you're an op in the channel.
If you run the network, you have more options such as various channel services that you can get that will add protections.
