In Warsaw I can think of at least 10 craft beer pubs
Ten? There are now more than fifty! ontap.pl/?city=warszawa&type=1
How much will be the investment approx?
How long is a piece of string and how much have you got? It all depends on what kind of place you want and where. If you want maybe eight taps in a biggish city and don't mind doing a lot of work yourself and having a pretty cheap interior (all furniture from junk shops etc), you could do it for well under ten thousand euro all in. If you want a twenty tap stunner in central Warsaw in a custom converted location, you'll need more than ten times that amount.
Personally I'd stay away from Warsaw, there's just too much competition here now. Wroclaw might be an option, or perhaps Gdansk but both have increasing competition and raising rents. Bialystok only has two multi-tap places, one is always packed and the other has prices at central Warsaw levels. Lublin has a couple of multi-tap places by only one is worth a second visit to, so perhaps Lublin might be a good place to consider.
Here's a list of the craft beer bars in each city in Poland: ontap.pl