There are people who don't know much about databases who would advise you that the figure is 10000, or 15000, or 30000 - in other words, whatever they think is a big number (even
one per shoot - doh! ).
Adobe don't publish a number, and I don't think there is any known limit. Anecdotally, my main test catalogue is 80k and runs OK on an oldish machine, but I often help a friend whose catalogue was 210k last time I looked, and grows with up to 1500 keepers per week (he has a 5k a week habit). Again, it's not on the latest machine and is working quickly enough for a very busy pro.
LR does stretch a lot of aspects of any system, but I'd advise you to see how it works with a million images - and report back!
John