I have set my Netgear router to email me about DoS attacks & Port Scans.
Do you enjoy deluges of emails ?
The source varies, but many seem to originate from China.
Anyone else had a huge increase?
"Seem to originate" is the operative word...could easily be virus infested zombies under C&C.
20 per day = "huge increase" ?? It's automated portscans by script-kiddies. Maybe your IP range has come up for scanning in their lists again.
I’m not worried (should I be?)
Not much you can do about it. It's a fact of life on the internet. Practice safe computing, keep everything patched up (including the router), review the router config once in a while to make sure nothing has been tampered with (at least obviously tampered with that is !). A cheap Netgear router probably doesn't have much ability to do much else apart form be a bystander.
What you need to look out for is the targetted DoS attacks that eat up your bandwidth .... anything else is just the usual internet "noise". But generally any half intelligent ISP will cut off customers who become bandwidth sucking targets in an "ask questions later" policy style.
Put in additional layers of security if you are worried. Or if you want to keep yourself busy on a rainy day, report them to their ISPs abuse department (just don't expect any magic action to be suddenly taken).