Concerning amperage/voltage, remember volts is the “pressure” the electricity flows at, like a high pressure hose trying to blow up a balloon, it can “burst” something not equipped to handle it. Put 12v into something that takes 5v, and you get the wonderful magical blue smoke. A couple volt difference usually is immaterial, but try to get it spot on.
Amperage is the “amount” of electricity that flows in a given period. If you have a lot of LED’s on a strand, you need a higher amperage to get that larger amount of energy to them. If you underrate the power supply, it will still TRY to provide that power, overheat, melt down, explode, go nuclear, etc.
You can have a power supply rated at 20amp running a strand of LED’s that only take 2amps, and no harm will be done, it just won’t be using the power supply’s capabilities fully. If you sting 2 or more 5m strands, you’ll want 3-4amp, 5 to be on the safe side.
Do not plug AC power into these, they go boom, see a photo earlier in the thread of a fella that did just that.