How many notes can you fit into 20MB? That depends on the kinds of notes you want to export. If most of your notes are text notes, you should be able to export approximately 200-300 notes at a time, depending on the average length of your note. If most of your notes are scans or drawings, the file sizes for those notes are larger, so you'll be able to export fewer at a time.
In the event you try to export a batch of notes that is too large, Insight Notes will tell you. If that happens just select a smaller number of notes and try again, or subdivide the batch into two or three exports. If you are a backup subscriber, you can make as many exports as you wish.
We encourage all backup subscribers to periodically export their entire notes database to themselves for extra redundancy.