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: 
If you want to get it really right, then you need to say:

* To one woman: as-salāmu ʻalayki (or ʻalayk)
* To two women: as-salāmu ʻalaykumā
* To three or more women: as-salāmu ʻalaykunna (or ʻalaykunn)
* To a mixed group (men and women): as-salāmu ʻalaykum
بسم الله الرحمن الرحیم

Kum (کم) means plural you (‘men’ or 'men & women')
Kuna (کنَّ) means plural you (only women)

Yet in conversations it is common to say Alaikum, however if we are to speak with eloquence to an only women group, then say Alaikuna is more correct yet uncommon, especially in non-Arab dialogues.