Question: Normally we say "Assalam o Alaikum", but my sir said that for women we need to say "Assalam o Alaikuna". Is that right?

Answer: 
In regards to greeting in Arabic (for females):

According to Arabic grammar, KOM is used for men, and Kon (Konna) is used for women. As a result if you’d like to use the correct grammatical shape of it, you ought to say “Assalam o Alaikon (Konna) which is written like the following phrase:

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> السلام علیکن
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> 
> 

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Of course, there is another nice point, and it is related to singular and plural nouns. Actually “Kon” is related to plural noun (according to the correct Arabic grammar).

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But is the singular noun it is writes:

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> السلام علیک / Assalam o Alaike (For female)
> 
> 
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As much as I see, it can use for both men and women in informal form. But if you are looking for the formal form of the greeting or saying salam for women, you should be aware that if there are more than 2 women, you should say Assalamo Alaikon.

There is another important matter that you can note, and that is about two women. In fact if there are two women, you should say Assalamo alaikoma in the form of formal and in fact according to the true grammar of Arabic.