LegendarySSJ7

(Volume 24 / 8, page 52) Vegeta: "They say a Super Saiyan appears only once in a thousand years... I've always said it was just a myth... and if I was sure that even if it were possible........ the only warrior who could possibly become one... was me!" Only once in a thousand years, not once every million years. Whether it was one or multiple Super Saiyans coming out of the legendary fold every millennium is never stated, with only vague, aloof words. This is never contested - the whole reason why Vegeta even sees past the state; this is the notion of surpassing it, as pronounced after #18 defeated him - nothing jars with the notion of the status being Legendary. Notice how I say surpass and not master as no one was out to master it yet (until Goku sagaciously peered into said option).

To add gravity to that statement, each new form is achieved as a response to any new threat - hidden, metaphorical, or blatant. It's SS Goku in the Freeza Saga Arc, it's SS2 Gohan in the Cell Saga Arc, and SS Vegetto in the Boo Saga Arc (all of them coming out on top as the strongest). As for Nappa and Raditz, not to chide, but they weren't anyplace close to ascending the state. Not even Vegeta in the Freeza Saga Arc could do it - he had to break the 3,000,000 barrier and find the right motivation (as well as a pure heart, evil or good). Aside from that, canonically, Goku is the Legendary Super Saiyan, and nothing jives with the statement.

__________________