What are the current trackers I need to buy for FBT for the Index?

There are no Valve trackers, as far as I can see, so I have to buy the HTC Vive Trackers 2.0 (2018), right? So, these below? 
https://www.amazon.de/HTC-Vive-Tracker-2-0-Virtual-Reality-Verfolgungssystem/dp/B0748ZY323/ref=sr_1_3?dchild=1&ie=UTF8&keywords=vive%20tracker&language=en_GB&qid=1584559730&s=ce-de&sr=1-3

How many do I need to buy? Two, three?