You can working on your game now already (https://itch.io/jam/neo-twiny-jam/topic/2858806
All text in the Story page (including the text inside the links/buttons to go to the next page counts in the word count (even if it just say Next/Continue).
Menu (like settings/credits) or an Intro page does not, as long as it does not include playable/story content.
You can always add onto the story later on:
https://itch.io/jam/neo-twiny-jam/topic/2836658/