If I were to say "Five grandmothers", should I say it as "Nyanya watano"?
What made you think of grandmothers? Do they use the same word for grandmother? Or were you just merely playing with declination regardless the meaning?
Depends on where you are. In Swahili it is easy to create a lot of confusion between grandmothers and tomatoes. I forgot the details but is explained in one of the comments where the word bibi is introduced.
The word Bibi, is much more common for grandmother, so you could also say Bibi watano.