Bikes come and go. The only time I've ever regretted selling a bike is when I sold a clean rare bike to someone that beat the shit out of it. Even then, I didn't regret selling it, just selling it to that person.
You can't ride them all. Storing them sucks. The more bikes you have, the harder maintenance becomes on all of them, and the more likely it is that you're harming some bikes through neglect.
That being said, I have a bike or two I'll never sell. I think there are exactly two types of bikes in terms of emotional value; never selling and everything else. You know the bikes you'll never sell, if you're debating it at all its probably a seller.
When slimming down I usually start with bikes I have had for the least amount of time and have done the least with. Those are usually bikes I bought for cheap and thought would be a fun project.
The value you put on them doesn't really mean anything when even competitively priced bikes sit on cl/offerup for months at a time. Eventually you'll realize how much you care about a bike when you drop the price to $350 just to get it the fuck out.