I see a lot of $10Ks out there. Seems like a common number many folks use.
Of course, the question on the other side of the coin is "What kind of lifestyle do I want to lead and where?"
There's a blog/site called retireearlylifestyle.com that I highly recommend if you want to see how wisdom, frugality, and the willingness to take a different approach to early retirement has been going on for 20+ years, before concepts like FIRE were main stream.
I'm a Dave Ramsey fan and we recently became Baby Step 6a Debt Free...all loans and personal residence paid off, but we still have rental loans. Working on getting rid of those ASAP. That's the "6b" part. Then we'll be truly debt free!
I don't see a reason to get 300 doors if I'm content with 30. For that matter, 300 doors are 10x the hassle factor, and if the idea is financial FREEDOM, it seems silly sacrifice the 2nd word (FREEDOM) for the sake of the 1st (financial). I'm not financially FREE if I have to constantly be one step ahead of a market downturn and have $2-3 million in debt hanging around my neck. "Good debt" is only "good" until life doesn't work out perfectly as planned; then it causes people to go bankrupt.
So maybe the best idea is to start with the end in mind. What would I do with $10,000 / month? Could I be happy with $8,000? How about $7,000? What are my plans other than volunteer for a local charity, go fishing, hiking, or roaming around the world?
I could live a pretty satisfactory, full life on $3,000 / month in my medium size town (pop 160,000). With no debt, there's not a whole lot to spend money on that's worthwhile, in my view. I don't really need or want to own a Corvette ZR1, I hate golf, and a wine cellar probably gets dusty after awhile and not that interesting. I don't need to live in a prestigious neighborhood in a "hot market" and go to trendy clubs just to make it rain $1,000 bottles of champagne.
Who am I trying to impress, after all? Maybe living with contentment is a good goal. Having enough for today and probably tomorrow too is definitely a "freeing" thought!