Data Tinker
data-tinker / custom and edge values / number with n digits
Number with N digits generator
A custom value, as in the rules. The parameter: how many digits (6).
Free, no sign-up and no limits. Everything is generated in your browser: nothing is sent anywhere.
What if it filled itself in?
With the Data Tinker extension there is no copying and pasting: it fills the whole form from a panel next to the page, with templates per form and rules for your own fields.
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Profiles
Personal documents
Banking
Addresses and phones
Justice
Property and utilities
Vehicles
Dates and times
Texts
Technology
Test data: valid in format and check digits, but made up. A randomly generated number may match a real person's; do not use them on real systems.