Translation:There are not very many ponds in the city.
In reality, "piscina" was used for all kinds of artificial bodies of water, even water tank (for drinking for instance) according to Gaffiot. It wasn't for swimming, it was for all kind of uses.
They translate it by "pond", because it can be a swimming pool or a fish pond, but it can be really more, (and it was common use)
It's a mistake. They shouldn't accept "many" here.
The difference is: plurima > multa
great many/very many/very large quantity = plurima
..........many/..........many/........a large quanitty = multa
"Very many pools are not in a city" is less natural, in my opinion, than "There aren't ..."
My mental categorizations are based on a language (Hungarian) that doesn't have separate words for "lake" versus "pond", so I know the deliniation can be a whole lot less precise than that. Lewis & Short gives the following main translations for lacus: I. basin, tank, tub; II. a lake, pond; III. a large reservoir for water, a basin, tank, cistern. For piscina, the translations are: a pond in which fish are kept, a fish-pond; a pond for bathing or swimming, a basin, pool; a flood-gate, sluice, lock; a cistern, tank, reservoir.
I don't remember seeing any mention of swimming pools in any of the roman ruins, so that would be an interesting translation, and in my readings piscina was always a small pond in the garden area of a house, so didn't think piscina could be translated as lake, but guess not important. thanks again for your insight