You only require 1 bacteria to make yoghurt but lots of time. The more you add the faster it will complete.
I have used supermarket yoghurt as a starter and I have used probiotic capsules, typically around 5 billion CFU. The first batch takes a very long time to complete but using some of a previous batch completes much quicker. It just goes to show that a well fermented yoghurt has considerably more bacteria than most probiotic capsules!
For example 1 5 billion CFU capsule rehydrated and then placed in a 6 floz yoghurt jar, yes I know my yoghurt maker has very small jars :o( takes around 14 hours to complete ie make a set yoghurt using skimmed milk powder at a high concentration. Whereas using about 3 floz from a previous batch to make 7x6floz jars takes around 3 to 4 hours to completion. Yes that is 5 billion in 1x 6 floz jar and 3 floz spread over 7 jars. It's that much faster using the previous batches. I do add some FOS to it as well.
I recommend that you do it similar and only start afresh using the probiotics when you need to refresh the bacteria. I repeat using the previous batches as a starter for around 10 rounds over about 20 weeks. Bacteria can be affected by Bacteriophages so it is best to refresh once in a while but don't be too strict with this as most likely they will be okay.