N+1 is the number of <things> you need.

Bikes, snowboards, surfboards, guitars, etc

where N < the number where your spouse leaves you.

Spouses come and go. A limited edition PRS is forever. Besides it is probably going to end up a collectible and increase in value. Can't afford not to buy it really.

Without another adult in the household, how can I reasonably justify needing a 2-car garage to store all my bikes?

That's easy. You watch a few FortNite videos on YouTube and slowly but surely end up with a small motorcycle collection.

You're going to want to work on your own bikes and eventually fabricate your ow-- basically, you need the space for your $250,000 CNC machine anyhow, so..