Does every student at NIT in the CSE branch get a placement of more than 20 lakhs, specifically at MNNIT Allahabad?

You must be unaware of the ground reality of college placements.

No institute can guarantee a certain package to its graduating students. Let alone NITs, even the best CSE colleges (IIITH, IITB, etc.) can not guarantee anything during placement season. 20 Lakhs is a big amount in India, and even the best CS colleges don’t have an average package of 20 LPA (rather the figure is around 8-10 LPA). Very few students get a package close to 20 LPA. 

Do not get confused by the average package of premier institutes, as many of them convert international offers into Indian Rupees while making those calculations. Doing so makes the average package very high. These packages look high, but are very basic in relation to the living cost of the respective foreign cities. The same logic applies to MNNIT Allahabad, or any other NIT. 

Instead of focusing on the placement figures, choose a college with a good environment and high growth opportunities. MNNIT is a good choice for CSE, and it has a good record overall.