I just have two, if you're doing strictly u2f they are less than $20. I have a nano4 and a neo that I use for u2f, oath tokens, and rsa keys for sign/encrypt/auth. It's not the cheapest setup but it's highly functional and about the best account security I could put together. I lost a gmail account for 45 minutes once and decided that would be much worse then the cost of the keys.

I've dropped my YubiKey quite far onto hard ground, dropped it in water, and various other abuses. It seems quite hardwearing.

When I ran a CA, half of my root key to unlock the more sensitive keys was stored on an older Yubikey on a necklace and it never left my neck. That includes the shower and rolling over on it in bed. I couldn't make that thing fail, and arguably I wanted to based on how I treated it. (Back it up, though.)