Open the game on Roblox and read the Active number. To see the same count with its history, search for the game on RoRadar.
Open the game's page on Roblox. The Active number next to Favorites and Visits is the number of people playing.
The free RoRadar Discord bot posts a live player dashboard and spike alerts for any Roblox game.
RoRadar reads it from Roblox's public API. A page can be a few minutes old because it is cached, and each game page shows when its numbers were fetched.
Player counts change every second, and the two pages may have read the number at different moments.
Yes, for the games RoRadar records. Their game pages show a history chart built from hourly snapshots. Nothing is back-filled or estimated.
RoRadar does not report a total for all of Roblox. The player report shows a sample: the most played games in each genre.