Depends on the target site. With half a million routers you could cause problems to 99% of sites out there. The percentage of websites that could handle 500k concurrent connections is very small.

If we assume each router has, on average, 0.5Mbps upstream (hopefully it's higher!) then that's a combined capacity of ~250Gbps.

Some quick searching says the average DDOS size at one point in 2017 was measured at ~14Gbps and some larger attacks were peaking at ~120Gbps. Cloudflare's "biggest DDOS ever" was 800Gbps.

Even if we assume a lot of these routers are clustered on specific ISPs or networks and the effective capacity will be less, just on sheer bandwidth we're still well into or above the range of some of the larger DDOS attacks.

Whatever way you look at it, I'm sure 500,000 routers is enough to cause some trouble for most people.

A lot of DDOS attacks use UDP-based amplification techniques. I know DNS and NTP were frequently used and could get amplification factors of up to 500x. This year there have been some amplification attacked using Memcache that could get 50,000x amplification.

It's not so much as amount of connections but amount of small packets sent. I can handle 500k connections without problems with enough RAM. You don't need a lot of connections to DDOS someone, you only need a lot of small packets (~84 bytes) or in case of volume attack a lot of 1536 bytes packets.