First, the request for sequential numbering comes up ALL the time. There are MANY posts in the Community. I encourage you to search for them to gain more insight.
The short of it is....if you have multiple users entering these rows of data, it will be virtually impossible to keep the numbers sequential. Why? AppSheet is a distributed system - meaning each user has their own copy of the app and data. There is no way for the app to know that another device just entered a row with the next sequence. So two users write different rows with the same value. If that value is the Key column, the last submitted row wins, essentially overwriting the previous row with the same key value. The more users entering these rows, the more issues there will be. (LEARNING MOMENT - always, always, always use a dataless key column - one that is a random selected value AND not derived from any data columns. This means adding a column that is dedicated as the key column and not used for anything else.)
I would encourage you to first strongly evaluate the need for a sequentially numbered value. Why is it needed? The usual response is "to easily see how many rows there are". This can be handled other ways such as using views to group and count the rows for you. Or maybe that was just the way it was done in the spreadsheets before having an app. In this case then maybe it's really not needed but instead simply could be a UNIQUE value.
If you MUST have a sequentially numbered value, then the only accurate way to keep it in order is to inspect and adjust as needed after the rows are entered - i.e. with automation or simply as manual adjustments as needed if it doesn't happen often.
I hope this helps!
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