One echo and two dots. We don't end up using the big speaker (the echo) much for music, so we probably could have gotten by with just dots--but we got the cheaper early adopter price on the echo, and the dots didn't exist at that point.
Any of them function on their own.
I only recently added the dots because we have more "smart" home elements now. Philips Hue lights, power switches (e.g, I can turn the Christmas tree lights off from bed), television remotes, and I'm hoping Santa brings me a smart thermostat. It turns into an energy saver because I'm more likely to turn lights off in a room since I can turn them all off at once by voice.
(Or I tell Alexa to start up the television while I search for the remote.)
Naturally they are VERY good at helping you shop from
Amazon. If I say "Alexa buy cat food" she'll check my order history and offer to re-order recent cat food items. And this year they have run several deals that you can only get through Alexa, such as a television last week that was insanely cheap, like $120 for a 32".
I believe the dots can read back audiobooks to you, too.