We are planning for Heroku Platform migration...How to estimare dynos required for the application...?

Without details about your application stack, this would be impossible to estimate. Each Heroku application is run in a dyno, which is essentially a Linux container. You really need to read the Heroku reference documentation on their web site: https://devcenter.heroku.com/articles/how-heroku-works

commented: Thank you.,..will read about it... +0

We work for Marketing Customer facing application ... Heroku is chosen as platform to migrate...application is Java & Spring based and I see its one of the supported language/platform in Heroku....As we moving to Agile methodology, we see ready to use platform will ease things...& we can just focus on the application...But what could be some advantages & disadvantages of Heroku in the long run...

Good question that I can't answer since I haven't used it. I understand that Heroku is based upon a Debian/Ubuntu distribution however. There is a lot of useful information on their web site and the documentation seems pretty thorough to me.

Disadvantages: would mostly be whether or not it will survive in the "long" term - long being relative in the Internet era.

Advantages: it was purchased by Salesforce.com in 2010, so that may be a good sign.

Go to www.heroku.com for full information.