Having three plural forms for Slavic languages is typical, e.g. Ukrainian and Russian have them. Roughly speaking, one form for numerals ending with "1", a different form for numerals ending in "2", "3", "4", and the third form for the rest of them (simplifying a bit).

Slovenian appears to have a special word form for numerals ending in "2". Looks like it's a remnant of the dual number that existed in early Slavic languages: https://study.2tm.eu/blogs/the-dual-number-in-the-slovenian-... And it is indeed unusual!