A seat is a number of team members you can invite into your world.
All created stories/documents etc will be saved, you just won't be able to create a new one.
It is hard to tell, but for example Leviathan: The Last Day of the Decade was developed by up to 3 scriptwriters for 3 years and it has 119 Documents and 213 Dialogues. For an Indie game, a free plan is enough to create a detailed world and big story.
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