Unfortunately the taxis there are unregulated.

However, taxis from Taxi Stockholm, Taxikurir, Taxi 020 all have fixed prices from Arlanda. From memory it should be 520 SEK with Taxi Stockholm. See picture how it looks at Arlanda. There will be several lines clearly marked with the name of the taxi company. Don't be afraid to take your time to figure out which taxi to take. It would help to mention the fixed price to the driver.

Photo in this article https://www.svd.se/avreglering-gav-dyrare-taxi-och-flyg

There are several drivers around the city that try their hardest to look like they drive for Taxi Stockholm. Therefore you should always confirm that the phone number is written on the car:15 00 00.

If you don't want to navigate that there's Uber. There's also Door to gate which is a shared ride service like Supershuttle in the US.