I have had up to 6 vehicles at the same time,but I only had 4 of them registered at one time--that was back in my 20's and 30's when I lived at my parents house that then had 4 acres of property and hiding a few unregistered vehicles wasn't too difficult...but here your supposed to be limited to ONE unregistered vehicle per yard,and I am still in posession of two,thankfully no one has complained and the town code enforcement dicks haven't knocked on my door yet...(subject to change any day though,they are out looking for revenue and many guys I know have been told to "use them or lose them" or face a hefty fine..
I think when your young and single is the time to do what you please with cars and trucks,because once your married or end up renting an apartment ,you cant have all these "toys" any more...as you get older and your body revolts against you when you try wrenching like you could before,you lose interest in doing it,and if your spouse is always whining about the "mess" and expense your putting into old vehicles,your marriage could be in jeapordy..(one reason I nevrer GOT married!)...
I currently have two of my four vehicles registered and insured,that costs me 700+ bucks just for compulsory insurance,which is far too much for a guy who's vehicles often sit in the yard all month and rarely get driven...I have the '95 Ford Contour and my '82 K2500 GMC registered,but I do miss my full sized G-10 van a lot when its summer and I want to go somewhere far away and camp out with it..unfortunately there is no one vehicle that can "do it all"--I need a economical vehicle for long trips ,and a 4x4 truck to plow and get firewood with,and I'd like to have the van so if I want to carry more than 2 other people I could..unfortunately no vehicle I know of would fill all those bills,so I had one of each instead!--nowadays its just too dam expensive to keep more than 2 on the road,all their tires from going flat,batteries from going dead,and who has 80-100 bucks to fill each gas tank any more??...
How many is "too many"--depends on where you live,whether you can store them without getting cited or ticking off neighbors,and how much disposeable income you have...I got a lot of free vehicles and many for cheap when I worked in parts stores,often a customer would say "oh,its going to cost THAT much for the parts??--forget it,I am going to JUNK it and buy something else"!..if it were a GM car or truck,I jumped at it and bought them--many I drove a long time,but some I flipped for a very healthy profit in a matter of weeks,after my parents started complaining about the yard "looking like a used car lot or a junkyard"....