There appears to be no setting to have an entire document permanently set for the thousands comma i.e. 5,000 instead of 5000.
If I change or add a cell, it defaults to no comma.
If I change one cell to say 2 decimal places, it defaults to no thousands comma.
If I open a new sheet, it defaults to no comma.
I’d like the comma there forever. Please.
I assume that your question is related to Calc. You tagged your question as common but I think there is no answer for Writer and Impress.
You must define a user formatting code.
select one of your cells with a to-be-formatted number
Format>Cells, Numbers tab
in Format code, enter #,###.## if it does not exist already in the list (otherwise select it)
In case your number(s) may be larger then one million (billion), you need to specify the extra separator as #,###,###.## (#,###,###,###.##).
Select the cells to be formatted and apply your user-format to code to them.
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Even better, define the number format in a cell style derived from the Default style and apply that style to the selected cell range, instead of direct hard formatting.
In case your number(s) may be larger then one million (billion), you need to specify the extra separator
That is not true. The group separator will be repeated for large numbers.
@erAck: when I checked my format, the repetition didn’t occur (though I thought it would). Did I make a mistake?
I don’t know 
Make sure the comma in #,###.## is actually your locale’s group separator, otherwise it would be just a literal character at that position, which then would not be repeated for larger numbers. But then again it would also be displayed as a leading character for smaller numbers.
Thanks for the answers - but - the formatting doesn’t stay as a default setting for new spreadsheets. When I open another sheet, the default is NO thousands comma separator.
This is not an answer but another additional question to your original one. Please, edit your question to include your request to have persistent change.