Pricing
The pricing model for the Workout Bot is pretty simple. As mentioned in the documentation, there's a limit to the amount of different users that can use the bot in any one group chat. The reason for this is that for each new user that interacts with the bot, an individual account is created.
The subscription plans only differ in the amount of users allowed to use the bot on the server. Beyond the limit, new users won't be able to register their exercise unless the server is upgraded.
Unless you have a dedicated fitness server, the cheapest plan is probably enough.