so nanohertz is one wave per 10 years?

1E-9 cycles/second <--> 1E9 seconds/cycle

Figure 3.2E7 seconds/year so this translates to ~32 years/cycle

Is it wrong to assume there's many other wavelengths of these waves?

No, they come in a wide range of frequencies/wavelengths depending on the systems under study. LIGO is sensitive to a higher frequency/shorter wavelength range. LIGO can't access Nanograv's regime due to limitations on reducing environmental noise.

> The team found key evidence for quadrupole correlation signatures compatible with the prediction of nanohertz gravitational waves at a 4.6-sigma statistical confidence level (with a false alarm probability of two in a million).

99.379-99.9767% confidence, not bad.

If gravitational waves come from several sources, then are there rogue gravitational waves like in the ocean?

Another source described the ordinary gravitational waves as "deafening", so the rogue gravitational waves should be even louder.