I have never counted but I would guess somewhere between 20K to 25K??? I'm 58 and started collecting seriously when I was 8. Pretty much everything from 1972 on was purchased new and almost all of my key back issues were purchased during the 70's, except for Hulk #1.
There's a cool story there. I had complete collections of many silver age Marvel titles by the time I was 16 except for Incredible Hulk which I was only missing #1. When I was 16 (1980) a friend of my mother whose father (who collected comics and recently passed) left his entire collection to her and her brother. I met her a few times in the past and had told her about my collection. She knew I was an avid collector like her father and after he passed, she GAVE ME Hulk #1!!! I would conservatively estimate it to be somewhere between a 7.5 to 8.5 grade, but it (like all of my books) has never been professionally graded.
I've never sold even one book and I'm not about to start now. My daughter gets them when I check out.