Question

How many planets in the universe are like earth?

Answer 1

No one knows.

Explanation:

We are currently discovering countless planets orbiting other stars, called exoplanets. Some appear to be similar to Earth in size or mass, potentiat capable of holding liquid water because they are the right distance from their stars -- or perhaps both. Might one or more "exo-Earth", so to speak, have life or even intelligent life? No one knows ... yet.

Answer 2

If you look up the Drake equation (here: https://en.m.wikipedia.org/wiki/Drake_equation) you can estimate this yourself.

Explanation:

Given that there are billions of stars in the galaxy (typical estimate might be 2x`10^11`) most astronomers estimate there are a similar number of planets, though clearly not all are like Earth.

You then need to factor in the number of galaxies (estimates range through the hundreds of billions, but seem to keep growing) and you are up to a very large number, irrespective of how small the percentage of planets you consider to be "like" Earth.

The original answer was wholly true, but hopefully you've seen how we might estimate the number, even if we cannot measure it.