Is it possible to calculate the number of atoms in the universe?

within a few orders of magnitude, depending on how you want to define the word "universe". the number is pretty large. We can come up with a reasonable estimate of the mass, and that pretty well imposes the number of atoms, given that hydrogen is by far the most common element. The mass estimate depends on a lot of fairly uncertain parameters, of course (average mass of a galaxy, number of galaxies, those sorts of numbers), and obviously we cannot define what we cannot see, and we cannot see the entire universe. It is not possible to give a limit to something that has no evident end. And it doesn't matter at all whether you assume hydrogen is all mass, or only half the mass (or something else). the uncertainty elsewhere is far larger than a factor of 2.

It is just a standard back of the envelope calculation (rough estimates for all important factors will give you a rough estimate for the total).

Yes but it's gigantic.

There are more atoms In one human than there are stars in the entire visible universe.

In the Wikipedia article entitled "Observable universe" you will find a mass-based estimate using the assumption that all atoms are hydrogen: there would be about 10^80 atoms in the observable universe. If you then visit the article on "cosmic abundance of the elements" you find that in our galaxy, 74% of all the atoms are hydrogen, and a little calculation will show that those hydrogen atoms account for about half the mass of our galaxy. So the 10^80 atoms estimate may be inflated by a factor of 150/74; therefore let's say the number of atoms in the observable universe may be between 10^79 and 10^80.

Not exactly since we do not really know all the possible structures of atoms in the Universe.All our calculations are made relative to the earth as the frame of reference.Hence calculations are only approximations.Not all representnt reality.

No

One could calculate only a gross statistical approximation. For one thing the number is always changing.

No - because we don't know how big the Universe is, but we know how many there are in the OBSERVABLE UNIVERSE.

between 10 raised to the
84 and
89

Someone did that already. It must be 10⁶⁴ (1 followed by 64 zeroes) or so, if I remember correct.

It is possible to estimate that number, but not count them or calulate the exact number.

approximately yes and its already done by science