Why you cannot invent a RUC by hand
A RUC is eleven digits where the last one is derived from the preceding ten through a weighted sum modulo 11. Type eleven random figures and your odds of landing the right check digit are roughly one in eleven, so most of your fixtures will bounce off your own validation and you will burn time debugging a defect that does not exist.
The prefixes that signal taxpayer type
The first two digits declare which kind of taxpayer the number belongs to: 10 for a sole trader, 15 and 17 for specific cases and 20 for a legal entity. Generation spreads across those valid prefixes, so the data you get resembles what you will actually meet in production rather than a uniform sequence.
Wiring them into your suite
Request between 1 and 1000 numbers and download them as plain text with one RUC per line. That shape drops straight into a database seeder, a fixtures file or an environment variable for your integration tests, with no cleanup step required.