Ok, looks like Itch notifies your followers like this:
- New games or projects you release will be emailed to your followers
- New devlog posts, and any other account activity, will be published to your followers' feeds
- If a project of yours goes on sale then anyone who follows you and doesn’t own it yet is eligible for an email notification for the sale
So uploading new files won't spam them. If you don't want to fill their feeds with devlogs, maybe just add one devlog for the jam and update it every day with the date/work done?