How player counts are calculated?

PlayStation games stats (as of 1 March 2019)

Total number of PSN player accounts: ~357,000,000. On average, for every console sold there are two player accounts created.
It is in alpha, so don't expect frequent updates. Bulletstorm game page shows what it will eventually look like.
* Growth (1 Mar – 11 Mar) is experimental, for recent major releases and PS+ games, with a bigger margin of error.