One e-mail and you can opt out for all but the magazine and voting forms.
My experience was late '70s, early '80s, before e-mail. At that time there was no escape. Half of it wasn't NRA itself, but Neil Knox, ILA, Whittington Center, NRA mail-order merchandise, NRA credit cards, NRA travel, NRA insurance, NRA political candidates, etc., etc., piles of junk nearly every day. You can't put a spam filter on snail mail. I finally didn't renew, and the volume increased, until it finally petered out after maybe five years.
And that was back in the dark ages before algore invented the internet, and the big markets in addresses and email lists didn't exist.
I believe in NRA objectives, but I probably wouldn't join again if they paid me.