It's not hard to get around by adding a much larger value in the maximum field, but as it stands adding an integer and leaving the maximum field blank results in the form not validating (with the message returned "Please select a value that is no more than 1000000")
Again, easy to workaround, just seems like the 'leave blank for no maximum' isn't really true if that's how it goes.
@greenstick - Can you provide a little more context for this feedback?
It's not clear to me which field you are talking about or where in Backdrop CMS you are running into this issue.
Then try to add content to it, and enter a number that's more than 1000000 in the new field. The field will highlight red, and when trying to save, the tooltip "Please select a value that is no more than 1000000" will appear alongside it.
Again, putting a larger number than 1000000 in the maximum field fixes the issue, it's more of a wording issue with "...Leave blank for no maximum." If left blank, the maximum is a million, not 'no maximum'.
Does this highlighting happen before or after submit?
I was just able to reproduce it with firefox and a higher number.
Ooops, update: There is a hard limit in function number_field_widget_form().
I'd suggest to open an issue in the core issue queue. Add this link to the related code.