How many planets in the universe can support life?

Only one that we know of.

The only planet that can support life that we know of is Earth. The set of circumstances for a planet to be able to support life, things like temperature range, atmosphere, available water, and the existence of a magnetosphere are not all that unique to Earth. There are planets in our own solar system that have some of the requirements, but only Earth has all of them.

The distances between Earth and even the nearest star besides our own, are so far that although we can determine if those stars have planets and if those planets are in the right temperature range we cannot determine the availability of water on those planets nor can we tell if there is a suitable atmosphere or if there is a magnetosphere. So even though we have found a few planets that might be able to support life we don't know for certain if they could.

Drake's equation is an equation that took certain variables and assigned numbers to them to determine the likelihood that humans would find other intelligent life in the universe. One of those variables was the percentage of the number of planets that could support life actually having life. It is pretty much universally agreed by scientist that if a planet can support life, life will be there.

If you consider the hundreds of billions of star in our galaxy, even if only 1% have planets and even if only 1 % of those planets has the right temperature range and 1% of those had atmosphere and 1% of those had water and 1% of those had a magnetosphere, we would still be looking at 1000-4000 planets in our galaxy that had life. Now there are billions of galaxies in the universe. So even though the number of planets that can support life that we know of is one, it is very likely that there are billions of planets in the universe that can support life.