On average about 100 (TCP) requests hit my home router per minute doing various probing and scanning. Lots of checking for the telnet port obviously. Sometimes you can see a swarm of entirely different IPs scanning the full port range (probing the ports one-by-one).
You'll see a lot of deepfield, censys-scanner, visionheight.com, shadowserver.io, etc., but also the usual suspects of Chinese or Russian IPs.
With OpenWRT I use something like this: `tcpdump -i pppoe-wan 'inbound and tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn'`, or alternatively `tcpdump -i pppoe-wan 'inbound and tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn and not port 44000'`, if we have some torrent client running (e.g. here at port 44000) which would mess up the result. I'm not sure it's the best way to handle this, but it's definitely enlightening what bounces off on the router.