Changes to the data in your sheet—changes to cells, adding or remove rows—are synced to apps within three minutes, although sometimes these changes will happen immediately.
Changes to the layout of your apps—changing components, themes, or other settings—can take up to 10 minutes to appear in existing apps, but should be available immediately for new users.
Changes to your app icon or splash screen often will not appear in existing apps unless the app is reinstalled. Android and iOS save your app's icon and splash screen when you add the app to your homescreen, and often will not refresh until the app is re-added.
Changes that are added within the app sync back up to your sheet in seconds.
If you are in the builder and would like to see the changes you have made in your sheet immediately, you can hit 'Reload sheet'.
Yes. Glide scales with your app, so there is no difference if a hundred, a thousand, or a million people open your app at the same time. All of them will be able to access it.
Sheets in your Google Spreadsheet Doc become tabs in your app. There isn't a strict limit on the amount of tabs you can have, but currently the bottom tab navigation can only fit up to 6/7 before it starts looking a little squashed!
If you have too many tabs at the bottom of your app - you can add them to the App Menu which is an offscreen menu.