I think it all depends if you wish to obtain the absolute minimum you can get away with or the maximum to cover all eventualities and be a 'tackle tart' at the same time......
Persoanlly, I have two 10' #7's for resevoirs, several 9-9'6" #7's (I take alot of people fishing who dont yet have their own gear) a couple of 9' #6's for fun fishing in small waters and a 9' #5 for playing about when conditions allow and hopefully, one day, a spot of river fishing too

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I also have a 9 #8 for heavy resevoir fishing and SWFF , a #10 for SWFF and a 9' #14 rod, well, it aint for brook fishing now is it!?
I seriously doubt I could really justify it all but as someone who grew up with constant 'hand me-downs' I simply enjoy being able to say I worked for it and hence enjoy having my toys.
I also have a number of reels and mostly use two or three of them and generally only the floaters and intermediates.
It is horses for courses really and entirely up to you what you want and how much you wish to pay.
In my defence, I seldom buy new as I dont see the point as I feel I can get more for my money second hand or 'last seasons' models. It is cheaper and just as good and sometimes better (the old Platinum XD's are better than the new models IMHO)
So really, it is up to you. Everyone can justify their collection but do we always 'need' it? No, not unless you are a Comp Angler
Ben