The Safari Technology Preview update is available through the Mac App Store to anyone who has downloaded the browser.
Apple's goal with Safari Technology Preview is to gather feedback from developers and users on its browser development ahead of launch. Safari Technology Preview can be run side-by-side with the existing Safari browser and while aimed at developers, it does not require a developer account to download.
Top Rated Comments
So just like Chrome has stable Chrome, beta Chrome and Chrome Canary, Safari now has stable Safari, Safari Technology Preview and WebKit.
Excuse me if my explanation was off but that's the way I see it. I'm not Apple so I don't have the official story.
I have about >20 open tabs and have no problems.