Translation:אתה מספר אחת עשרה.

Why is "אתה מספר אחד עשר" not acceptable?

Because ordinal numbers are always in the female. יש פה שנים עשר אנשים ואתה מספר אחת עשרה. It's not a number of people, it's an ordinal number.

I meant, שנים עשר אנשים counts the people so is in the male form. מספר אחת עשרה is ordinal.

So after עשירי ordinal numbers don't have the yud or yud-tav in the end and sound like cardinal numbers?

If I remember correctly, ordinal and cardinals are the same for numbers 11 and up. The numbers 1-10 have different ordinal forms. Particularly the number 1. I assume the course will go into it in another lesson.