"Native speaker" is not a very useful term: it combines a lot of criteria (first acquired language, language you know best, language you identify with, language of your parents, language of your ethnic group etc.), and each of these criteria is further very fuzzy (e.g. I know plant names better in Ukrainian, but programming terms better in Russian, which language I know better? Competency is not a single value, ethnic identification is malleable and people can have several of these, etc.)

These criteria usually coincide in speakers of big languages (usually languages of [former] empires), so it's relatively easy to say who is a native speaker of Russian or English. There are a lot of people who fulfill all the criteria at once.

But they rarely coincide for speakers of smaller languages (usually colonised people). When most people are bilingual, it's often harder to say who is a native speaker of Ukrainian or Belarusian. Most people fulfill some criteria but not all of them.

So, the term "native speaker" is not neutral and not very useful.

Original statements that led to this discussion

>>Polish is the second most-used Slavic language, right after Russian and just before Ukrainian

>This is not exactly right regarding Ukrainian. While it is the official language of Ukraine, in reality... let's say that not all Ukrainian people are actually speaking it.

The language debate about whether Ukraine is third behind Russian and Polish does not get heated till somebody here proposes a Slavic language that would have more speakers than Ukrainian does.

Here you go, stats, you see that Ukraine has a 7m larger population than Poland, but it's already conceded that not everybody there speaks Ukrainian, putting Ukrainian into 3rd place. Are you claiming that 36 million Ukrainians speak Russian and not Ukrainian which would put Czechia in 3rd place with 10 million speakers?

Put up or shut up.

  Russia        143,500,000
  Ukraine        45,490,000
  Poland         38,530,000
  Czechia        10,200,000
  Belarus         9,498,700
  Bulgaria        7,265,000
  Serbia          7,164,000
  Slovakia        5,414,000
  Croatia         4,253,000
  Bosnia and      3,829,000
    Herzegovina
  Slovenia        2,060,000
  Montenegro        621,383

What Slavic languages are spoken by more people than Ukrainian?

Wikipedia says https://en.wikipedia.org/wiki/Languages_of_Ukraine (with a dozen other languages under 1% each) top two:

  Ukrainian  32,577,468  67.53%
  Russian  14,273,670  29.59%

  32   million Ukrainian as 1st language
   6.9 million Ukrainian as 2nd language