However if you go for load balancing and reliability then you have to think of more steps in your environment: i.e.  two mx records, more than one SMG (a bundle of SMGs working together).

We are planning to use 2 SMG for inbound/Outbound

That's fine. And one holding the databases?

Both will act as separate SMG

Why separate SMGs?

So if you have larger environment my idea is to do it in this way.

smg1 is holding all databases (configuration, quarantine,tracker, statistics)

smgin1 (maybe add smgin2) for inbound

smgout1 (maybe add smgout2) for outbound.

If your intention is to check mailboxes directly via imap then add another smg for imap activities

In this environment all services report to one server. It is easy to move services from one smg to another one. It is easy to check tracker information or read quarantine, because there is only one database.

kindly tell me abt smgin1 and smgout1

is my understanding correct 5 smg servers for this environment 

Separate SMG means if one node goes down it will not affect the mail system that why 2 node smg

Yes, my suggestion above is to run 5 or 6 (6th for imap) smgs. But you can break it down to three instances (database plus two scanners for in- or outbound). And of course you can run your whole environment on only one smg; I have a lot of customers doing it in this way. However development does not see a problem to service large environment wit only one single smg.

My larger customers are using more than one SMG. But in larger environments all data is stored in one single database. If you do not trust a database on SMG than you can use your professional database in the background! So you have a lot of options to support reliability.

Yes, my suggestion above is to run 5 or 6 (6th for imap) smgs. But you can break it down to three instances (database plus two scanners for in- or outbound). And of course you can run your whole environment on only one smg; I have a lot of customers doing it in this way. However development does not see a problem to service large environment wit only one single smg.

My larger customers are using more than one SMG. But in larger environments all data is stored in one single database. If you do not trust a database on SMG than you can use your professional database in the background! So you have a lot of options to support reliability.