In 50+ years of theatregoing, I've been in every theatre many, many times, including some of the defunct ones (the Hudson, which is still there but no longer used for Broadway shows; the Mark Hellinger, purchased as a church when no one was paying attention; the sadly departed original Helen Hayes, Morosco and Ritz, all torn down for reasons of greed; the Henry Miller's, whose facade still exists as the Stephen Sondheim). There are probably some I'm forgetting...
I'm missing the Jacobs, the Wilson, and the Friedman (though I do have plans to see Casa Valentina, so soon I'll be able to take the Friedman off the list).
I'm still missing the Ambassador. Saw Chicago in the Rodgers and the Shubert. Saw It Ain't Nothin But the Blues in the Beaumont. (Before Blues, I was a teenager and a sporadic theatergoer). Between them, the show I'm truly sorry I missed was Topdog/Underdog. But those were some lean financial years. And I refuse to go back to Chicago just to check the theater off. It has to close SOMEtime.
I am proud to say, that in the 5 years since I started seeing shows on Broadway, in perticular, I have been in all but 1, the Helen Hayes. I saw Rock of Ages at its original home of the Brooks Atkinson, so that is gonna keep me out of there til RoA closes, though I have considered seeing it again just to finish off my list. Only recently though, did I finally check off both the MInskoff and the Gershwin (local school group saw Lion KIng, and they had extra tix, so I snuck along free of charge, and finally won the Wicked lotto to see Lindsay Mendez as Elphaba.)