There are roughly 190k practicing dentists in the US [1]. There are approximately 2.2 physicians per 1000 people in Poland and 2.5 physicians per 1000 people in the USA [2]. There are 38M people in Poland and 319M people in the USA [3].
Starting with the same density, (190k / 319M) * 38M, we would get 22.5k dentists in Poland. We know there are slightly fewer physicians per capita in Poland though. So let's take 22.5k * (2.2 / 2.5) and we get roughly 20k dentists in Poland.
Actually I think it's just a difference in definitions. You were looking at practicing dentists, while the number I quoted was licensed dentists. If I'm interpreting the numbers correctly, there are only 13088 dentists actually practicing dentistry (I don't speak polish, so I'm not sure if the difference is explained in the report). I'm also not sure how the world bank data is derived.
And doing even more analysis, there were 87687 practicing doctors (not including dentists) in Poland in 2014, i.e. 2.3 per capita, which is close enough to that it could have been World Bank's data (which isn't available for 2014). Comparing those numbers to the US it just seems that Poland has a smaller dentist/doctor ration than the US.
Starting with the same density, (190k / 319M) * 38M, we would get 22.5k dentists in Poland. We know there are slightly fewer physicians per capita in Poland though. So let's take 22.5k * (2.2 / 2.5) and we get roughly 20k dentists in Poland.
1. https://en.wikipedia.org/wiki/Dentistry_in_the_United_States
2. http://data.worldbank.org/indicator/SH.MED.PHYS.ZS
3. https://www.google.com/publicdata/explore?ds=d5bncppjof8f9_&...
Edit: Off by a factor of 2. Darn!