I think the right amount is, with what you can get on the walls and store, without it taking over the house. Or you having to move to a bigger house?

? OH, ER......I have just had to move house, so forget that....The right amount is the amount you are happy with.
Mind I did the opposite of you guy's, I only collected Horror film posters, but as time went on I ventured out into other fields, comedy, war, western, etc I even have Panto, and Circus posters now. Though I do agree with "only collect what you like" I could not collect for investment. The price hikes in the last 10-15 years are pushing out serious collectors, and are focusing on Investors. I do hope there will be a big crash, and I'll be able to get those posters that are now out of my reach.