Modules are per channel, not per bot
This is the part worth knowing before you plan anything. You add the same bot to each channel and turn modules on separately in each. A strict group can run filters and flood limits while an announcement channel runs only scheduled posts, with one bot behind both.
What your plan allows
The channel count is the thing plans are measured in: one on free, three on Starter, ten on Pro, unlimited on Business. It counts channels and groups you manage, so a community with a chat and an announcements channel is using two.
When you genuinely want a second bot
Identity. A bot's name and avatar appear in every channel it is in, so a brand that runs two unrelated communities usually wants two bots. The other case is handing one community to a different team without giving them access to the rest.
Neither case is about rules, because rules are already separate per channel.
The mistake that costs a slot
A storefront occupies one of these slots too, so a seller with a shop and one group is already using two. If you cannot add a channel and the count looks wrong, check whether a shop is holding a slot before assuming the limit is a bug.
Keeping it manageable as it grows
Name your channels so the dashboard list stays readable, and keep one list of which modules you expect on which channel. Four channels with differing settings is where people start forgetting what is on where, and the analytics per channel is the fastest way to catch a module you meant to enable and did not.