The largest record collection I ever saw was owned by an ex-girlfriend's father. He made $$$$$$ and lived in a huge house. The basement was his domain - it was completely soundproofed and I believe he had Klipschorns powered by Mac equipment with one seat positioned in the sweet spot. I would estimate he had about 6,000 - 10,000 albums in custom made floor to ceiling cabinets all around the room. It was quite a breathtaking sight and this was back in 1973 and he was still pretty young.
His daughter had a rock collection of about 600 albums which he bought for her.
The doors to the basement were like the ones in a broadcast facility - thick, heavy and sound proofed.
It left quite an impact on my 'young' mind.
Another friend, Art Collins, an A&R guy at Atlantic and Chrysalis had a huge collection also.