You can’t really answer this as a percentage, because it’s different if you have $300,000 or $10 million.
As a general rule (and with exceptions) the “3 to 6 months cash” advice is to get you past personal issues: you lose your job, your house burns down, your uninsured car goes into a lake, you have a medical issue and have big deductibles or uncovered expenses, and so on. That’s 3-6 months at a minimum and most everybody should have that.
If you have a lot of money, then I’d counsel to have a couple of years’ worth in some sort of liquid form. If/when the market tanks, sometimes it bounces back quickly (2020), and sometimes it climbs out of the hole more slowly (2008) and occasionally not at all (1929) at least for a retiree’s lifetime.
The idea is to prevent you from having to liquidate stocks to cover expenses while the stocks are possibly underwater, so you’ve invested $10,000 to get $5,000 back to pay the mortgage. Some stocks are less vulnerable to downturns (utilities, etc.) and some are wildly volatile (meme stocks, some tech) and a lot are in between.
There are other things that are liquid, like silver & gold, but they too are subject to swings. And there’s real estate, which usually doesn’t go down, but depending on the market, what you bought and where, and general conditions sometimes does - but real estate is never liquid, unless you buy a REIT which is very very much like a stock.
Do you have $300,000? Then have 6 months cash and hope for the best. Do you have $10 million? Then keep a few or several hundred thousand out to cover the NetJet fees and the servants, and find something good to invest in with the rest.