In standard modern English (both BrE and AmE) 'weren't' is one syllable and it rhymes with 'burnt'. An English-speaking child who is still struggling with their spelling might write 'weren't' as 'wurnt', because that's how it "sounds".
There are accents where it approaches two syllables, though. I don't know about N.America, but I do know that many Scots accents pronounce 'weren't' as two sounds. The Scots pronunciation could be transcribed as 'wurrunt'.