Sequential number is not officially supported with AppSheet, but this easy tricks could help some particular business requirement where the app creators need to generate the seq custom number or ID based on the rows, although there would not give much of flexibilities for numbering.
Use Googlespreadsheet Expression of
Even we have concurrent users, the added new rows will assing ID under usual [ID] field using uniqueid() expression. But spreadsheet formula will give the sequential number.
Even after row is deleted explicitely from app, the given row number will stay the same. But when we delete row from the spreadsheet directly, the expression will be revaluated naturally, then in turn give the new number.