How do I sort the list of invoices on iPad by either number or date?

Hello and welcome to the Community, michaelmadew-gma

On the iOS app, invoices can be filtered by Status (e.g. overdue) or Date range (e.g. last 30 days) by tapping the Menu > Invoices > Magnifying glass. 

For full sorting options including ordering by date, access QuickBooks on the web (QuickBooks is supported on the Safari app for iOS and Chrome app for Android).