@number

Grammar says that most nounbases, verbs, noun endings and verb endings must get exactly one of these three names —

singular (/ekavacana)
dual (/dvivacana)
or plural (/bahuvacana).

Each of these names is called a gender (/vacana).

Example.

When we say "the horse runs" using the nounbase **azva- अश्वॱ and the root dru द्रु, the nounbase **azva- अश्वॱ gets to be called a singular nounbase because it means one horse (rule dvyekayordviv... teaches that). Then it gets the noun ending /su, and not /au or /jas, because some rule (supaH) teaches that /su is added after singular nounbases.

Also, when we form the verb for "runs" from dru द्रु + /laT, some other rules teach that /laT must be replaced here with /tip, and not with /tas or /jhi, because this /laT shows the doer, and its doer, namely the horse, is singular.

Wait, didn't you just imply that **sapta- सप्तॱ "seven" is not a number?

To avoid confusion, I don't call **sapta- सप्तॱ a number in this website. I only use the word "number" to mean "one of singular, dual, plural". When I talk about one two three... I call them either "numerals", or "numbers" (with no at-sign @ in front).