To the cyclists this is never an issue but often it is for non cyclists.
However the formula for deciding this is n + 1 [where n = your current number of bikes]
The reason why this came up was that I was at a loose end on a sunny afternoon and decided to wash my bikes and my wife asked asked the question. I dont know why, as she has asked it many times before, so she should know the answer by now.
Here's my "stable".
|Dawes Super Galaxy|
|Genesis Croix de Fer|
Only one of them is "new" and thats the Genesis, built up in 2015, the Dawes is 1994, the Raleigh is 1980 ish, and the Boardman was cobbled together from bits and pieces.
The top two get regular maintenance as they're used for long rides and the Raleigh get taken out on dry sunny days.
The Boardman is the workhorse, and is used on a daily basis for shopping etc and gets taken for granted, so when I washed and cleaned it I found that 1. the front wheel bearings were loose and 2. so was the bottom bracket. After the wash it got a bit of TLC to make it right again.
The moral of this post is to regularly wash and clean your bikes as its the best time to check them over for wear and tear and to spot if any bits are loose.