For a personal discord bot, you can definitely run just one Machine, as long as you know there’s a risk of downtime if there are any host issues.
There’s some brief discussion about how to handle 2 Machines for this type of app in this thread:
By default, apps have auto start and stop enabled, with minimum machines running set to 0. This means that when your app has no traffic, the Machines will be stopped. This can be a good way to run 2 Machines without incurring the costs of keeping them running all the time, but it could be overkill for your needs getting started.
Here’s all the details about how auto start and stop work, which should answer most of your questions: