Fourth. While Ukraine is a bilingual country, it still much bigger than former Yugoslavia. Most people there consider it their native language and a majority know it. So if you are strongly biased for a small number you'll get 30 million speakers, if you are biased for a big number it might be 50 million.

And on top of that you must use the same criterion for the S-C language, not counting Slovenians, Albanians, Macedonians, etc who know the language but don't use it much anymore.