I think it mostly a matter of personal preference. In my experience most of the flavors don't contaminate the next flavor much with some exceptions, namely menthol and an overpoweringly strong homemade batch or raspberry I made. I personally use one atty until it dies usually don't clean it at all and I rarely even change carts, I just top them off and drip on the atty so I have no problem mixing flavors. Plus some flavors mix with others quite well such as cream and RY4 or vanilla with just about anything.
Some vapers go the exact opposite and have a single atty per flavor with dedicated carts and NEVER mix liquids other than when making a custom DIY liquid.
My starter kit was ok, but I am upgrading as soon as the post office delivers my PV today or tomorrow, so from experience, I would recommend to not go overboard on buying too many attys at first (is there such a thing as too many attys?). Make sure you get a few backups, but you may find that you don't like the device you're purchasing and want to switch to one with a different atty. Then you'll end up having a bunch of extra attys that you don't use sitting around.
If you want to experiment a bit with keeping the flavors pure vs. mixing if you only have a couple juices, your backup attys are sufficient to play around with to see which you like better. The worst case scenario would be that you may have to clean a few attys if you don't like mixing. After a good cleaning, I've never tasted a mixing of flavors.