666 is the number of the beast's name. nobody has any clue what this actually means, but it will become self-evident at the time.

it is a separate thing to the mark/sign/ tattoo of the beast, which again may be a separate thing from what people will use to buy and sell (this could still be cards, phones etc. but not cash, obviously.)

unless it forces you to do otherwise, read the bible plainly, without trying to extrapolate into it things that aren't there. that's how wars get started ;-)