"Five hundred and fifty-five fish is a good amount."

Translation: Tōngār se tōmys tōmēpsā klihossa sȳrior verdon issa.

[Ctxkh1Me]

I think I basically understand the reasoning, but why is issa used here versus issi? I can't think of many example sentences where there is a copula linking nouns of different pluralities; aside from this, I can only recall

Bȳrgār se tōmyz ampā azantyssy dārys sosy daor! "Six hundred and fifteen knights are not a king!"

In contrast to here, that example uses the plural form of sagon.

My assumption is that here the point of interest is not the fish but the count; a number is singular, so issa is used. Whereas in the other example the focus is on the plural knights. Any thoughts?