Hmm, does it mean some/several men were arrested? "Men were arrested" sounds a bit strange to me. But I'm not sure. I'm not native in English. Any thoughts?
I am a native English speaker and it sounds a little odd to me too. As you say, there would normally be some indication of number, e.g. "Three men were arrested..." Probably best to just think of it as practising a pattern rather than a sentence you will hear in real life.
Because (unlike English) Welsh always uses the third person singular of the verb with plural nouns.