I know several people who have come from the Arabic countries like Syria and Saudi, and have multiple wives. Is it allowed to legally have multiple wives in any country today, or do they only consider a single marriage certificate?
Not so simple. Marriage is administered by individual faith groups in Israel. I don't know how it's handled for Muslims there.
For Jews arriving from polygamous countries, my understanding is that decades ago the Israeli rabbinate would make exceptions, but they're approaching the point where they'd actually tell a Yemeni immigrant -- "you have to choose one of your two wives."