What is CRNM??

The RNM is the "Registro Nacional Migratório", the National Migratory Registry, Brazil's register of all immigrants, maintained by the Federal Police on behalf of the Ministry of Justice.
The CRNM is the "Carteira de Registro Nacional Migratório", the ID card that all immigrants in Brazil must have to show that they're registered in the RNM.  It's issued by the Federal Police.
Under the old immigration law, the RNM was called the RNE, "Registro Nacional de Estrangeiros", or National Registry of Foreigners.  The CRNM was called the CIE, "Cédula de Identidade de Estrangeiro", Foreigner Identity Card.  Most people referred to the CIE as "RNE" also, though.  You may have heard these old terms, since many people still use them.

I got one number named "CRNM" in my Protocal paper. Should I get the RNM while renewing my protocal?

If you have your Protocolo, you're already in the RNM.  Your CRNM should arrive at the Federal Police office where you applied for it before your Protocolo expires.  They'll contact you, probably by email, when they have it, and you can go pick it up.

Can I travel outside of Brazil using this RNM like my home country Banglades?

Probably not.   It's your national ID here in Brazil, where you can use it for all the purposes that a Brazilian citizen uses the RG, except for voting, of course.
Some people have reported being able to use it to go in and out of other Mercosul countries; I'd still always have my passport with me, just in case.

CRMN is just entering BR. Even when I had protocol # awaiting card, I still needed passport to enter AR. The card does not take the place of a passport.

Actually I didnt mean that I want to use this RNM card instead of my Passport.  I just meant that will I be able to come back in Brazil if I go for a trip to my home country. Will there be any problem in Brazilian immigration while entering in Brazil with Passport and RNM?

There should be no problem with the Brazilian authorities, as long as you're not absent more than two years.
Some people have reported difficulties with airlines at check-in, looking for a current visa and not finding one.  Explaining what the CRNM is should solve the problem.