Since there is Only one God
Atheists think that there's one fewer. Polytheists think that there's more than one. So how do we justify the use of the number 'one'?
and we the theists acknowledge Him
Even if we assume that there's one God, how can somebody be sure that the god that they worship is in fact that God? Do all prayers and devotions end up in the same place provided that the one praying believes in just one God? Or is it still possible to worship a false god, even while stoutly insisting that it's the only god that exists?
And what's up with the word "Him"? There's the gender angle to consider. Why not "Her"? More fundamentally, why should our concept be personalized at all? Why should God be imagined as a giant person, modeled on ourselves? So even if religion ultimately refers to one single divine object, can we exclude the idea that it isn't personal at all? The Neoplatonic One pehaps. Or Brahman from some of the Upanishads.
Which of the religion followers is going to be with God in heaven ?
Assuming God for the sake of argument, is the goal of our religiosity really to end up in some transcendental place called 'heaven"? Many Hindus and Buddhists would question that one.
Having asked all those questions, I'll return to the original question of the subject line:
Why do we need so many religions?
I hope that it's obvious that questions like the ones that I asked can be answered in many different ways. Not all religions have gods. Those that do, needn't have just one. Gods can be imagined, conceptualized and worshipped in countless different ways. Gods needn't be imagined as persons, as giant humans. The ultimate goal, the summum bonum of personal religiosity, might be different in different cases.
So that's why its probably a good thing that we have many different religions. They provide us with multiple alternatives, with different proposed answers, to our questions. Given the likelihood that none of us really possesses the answers, multiple religions allow us to entertain what amount to multiple religious hypotheses.
The world is a bigger and more interesting place with multiple religions in it than it would be if everyone believed in the same thing.