Please I am in doubt about the translation of a sentence in my latin course book,could you help me?
Bis septem liberi Niobae,superbae reginae Thebarum.
Would it “bis septem” be an expression?
“two times seven” I guess it is fourteen 
Larger numbers often didn’t fit meter, so it was common practice to do fancy things with numbers, such as “twice seven” (bis septem), or something like that, to mean fourteen. The sentence is rather incomplete, but instead seems to be from some poem? “The fourteen children of Niobe, the proud queen of Thebes.”
actually it is 15 in latin on account of inclusive reckoning.
actually it is 15 in latin on account of inclusive reckoning.
Inclusive reckoning only kicks in when you are counting from something or to something. Calendar dates, for example. In multiplication there is nothing to “include.” bis septem is definitely fourteen; cf. Aen. 1.71,* 9.161** and OLD s.v bis.
Chris is right that this sort of thing is a typically poetic usage; however, this example is non-metrical and so I would imagine it’s just a textbook showing different constructions.
- sunt mihi bis septem praestanti corpore Nymphae
** bis septem Rutuli, muros qui milite servent