Why do Mahans say human birth is a very rare and special one?

One need not go and look into the scriptures to answer this question. Open your eyes and look at the world. In the Universe that we live in, we see the numerous life forms ranging from the tiny bacteria, the worms and mosquitoes, honey bees, ants, rats, rabbits, dogs and other animals, the ferocious lion and the huge elephant. Human beings are an elite and sophisticated creation of God and we are right in the center of the plethora of the Almighty’s creations.

Above us are elevated beings like the Devas, Gandharvas, Yakshas and other demi-gods and gods and above all, is the Supreme Deity namely Sri Krishna.

Those life forms that are lower than humans have neither knowledge about God nor a thirst to attain Him. Those beings higher above the humans in the hierarchy remain in the same state until the ‘Pralaya’.

It is we humans that are fortunate to have the knowledge of God and possess the capability to realize Him. Hence human birth is rare.

Is there any other religion other than Hinduism and Buddhism that talks about Re-birth?

No.

Some say that there are exactly 7 ‘Janmas’ (births). Some say that there are infinite. Which one is true?

The fact that one undergoes infinite Janmas is true. The 7 Janmas here denote the seven worlds - Three worlds on the higher plane, the three on the lower plane and the one in the middle, which we are living on (‘sapta loka’). If the three higher and three lower worlds are considered as a one each, then it is sometimes alluded to as 3-Worlds (‘tri-loka’).

Each of these seven physical worlds has an astral counterpart too. If that is taken into account, it is sometimes said as 14 worlds (‘eerezhu ulagangal’ in tamil, that is, 2 x 7 worlds).

A jiva can be born only in these seven worlds and hence the term ‘7 janmas’ has come into being.

Is ‘Punarjanma’ (rebirth) true? They say that the sins done in this birth is carried over to the next birth. Is it true?

Rebirth is very much true. Not only sins, but also merits (‘punya’) done in a birth will be carried over to the next.

Some people say that once we have human Janma, we will not go into lower forms until we get realization. Is this right?

No this statement is not true.

Do we take another birth?

If you chant the Mahamantra, there is no question of another birth.