There are 2 ways to interpret the question...
1. Of people that I know (i.e. not strangers), how many have seen me dick?
2. To my awareness, how many people have seen my dick (i.e. including strangers).
I'll answer both, and for both only estimate in person (not online).
1. I'd estimate several hundred. The numbers added up quickly when I went to college at 18, given that I lived in dorms for several years with communal bathrooms, so pretty much every guy on every floor saw my penis regularly. Add in the college gym locker room, where I was likely to see encounter other guys that I know beyond my floormates. Same with gym locker rooms in grad school and at gyms where friends went over the years as an adult. Add in all the times I've been naked with friends in hot tubs, shared hotel rooms, skinny dipping, nude beaches, etc. It adds up. Then, of course, there are sexual partners
2. This number is much, much bigger, when you start with the people from #1 and add in strangers. For the past several decades, I've mostly gone to YMCA's with open showers. This generally gave me access to other YMCA's, which I frequently took advantage of during years of work travel around the country. I'd guess that on an average trip to the gym where I used an open shower, probably 10 or so guys see my dick, so that alone would probably multiply out into the thousands. Add in a few trips to a nude beach each summer over a few decades, and figure a 100 or so people probably see my penis each time, that probably gets it up into many more thousands. Back when I was younger, I went to a number of sex parties, naked parties, and bathhouses, again bringing up the numbers.
I've never really thought it through before, but it's kind of hot to think that thousands, if not tens of thousands of people have probably seen my penis in person.