How many multiple algorithm coins are out there now?
It would most likely never be possible, but imagine a multi algo coin that used...
SHA 256- Support network via ASIC
Scrypt - Support network via ASIC
X11 - Support network via FPGA and GPU
BLAKE 256 - Support network via FPGA and GPU
Cryptonote - Support network via CPU / GPU
Momentum - Support network via CPU / GPU
It would be perfectly balanced from a mining perspective. You may say im a dreamer, but im not the only one
uhm....i don't think is possibile. Can a coin switch from an algo to an other and than return back without problems?
Sure, but not without difficulty and a really good reason. The coin can hard fork at some height in the future giving everyone time to update their wallets, and the new wallet and miners will have to use the old algorithm until the fork point at which time the miners would have to switch algorithms. If the old algorithm is regular and the new one is custom then external mining software and pools will break. Anyone mining using solely the wallet would be fine. Not a perfect scenario by any means but within the realm of possibilities.