Once per sentence is enough - if you're sure you've reported a particular sentence already, no need to do so again. Then you "just" have to wait until a course maintainer looks at it… which could take a while. Months, perhaps. And hope that they accept the correction (in which case you should get an email).
There's not really anything one can do to speed up the process.
When a noun is preceded by a number, or by any adjective indicating that there is more than one (some, many, etc), then the singular is used. The adjective (including numbers) makes it clear that there's more than one, so I guess using the plural forms is redundant. Use the plural when there's no quantifier: "Little birds are on that car," "Az autón kicsi madarak állnak." .