"Six is a number."
Translation:שש זה מספר.
You open Pandora's box, if you ask too much about the copula ;-) Well, when you count unspecifically without naming the object counted, you use in Hebrew the feminine forms, like here שֵׁשׁ. On the other hand, you have מִסְפָּר, a masculine noun, as the predicate. In general declarative sentences, the use of זֶה is strictly casual. It agrees for gender, not number (when the predicate is plural, זֶה remains uninflected) with its predicate, not its subject: הַבַּ֫יִת שֶׁלְּךָ זֹאת דֻּגְמָא טוֹבָה your house is a good example (where בַּ֫יִת is masculine, but זֹאת agrees with the feminine דֻּגְמָא). Therefore זֶה agrees in this sentence with the masculine מִסְפָּר too!
Yes. This sentence has a subject and an object. Unlike English in Hebrew it is possible to form a sentence without a verb but there still got to be something that connects between the subject and the object. (Note that if we replace the word "מספר" with an adjective then you can omit the 'זה')
Well yes, בְּסַךְ הַכׇּל גַּם שִׁשָּׁה זֶה מְסְפָּר All in all, the masculine form of the six is a 'number' too. This sounds a little cheeky. But you could say this in contexts, where the masculine form is triggered: יֵשׁ לְי שִׁשָּׁה כְּלָבִים· שִׁשָּׁה זֶה מִסְפָּר גָּדוֹל I have six dogs. Six (ie. dogs) is a big number or שִׁשָּׁה זֶה הַמּסְפָּר הַנָּכוֹן בַּבַּשְׁבֶּעַ הַזּה Six is the right number in this crossword (because it ends there in ה).
Well, I suppose when you use pure numbers like inשָׁלוֹשׁ הוּא מִסְפָּר רִאשׁוֹנִי three is a prime number, you add mentally the word the number before it, which makes it masculine. You can see this usage (may one find it logical or not) on the Wikipedia pages dealing with pure numbers too, like here 5 (מספר), where you find it even before a feminine predicate: חָמֵשׁ הוּא סִפְרָה five is a digit.