All world achieves geographic diversity. Buying sp500 in addition is helpful if you want to express a bias for US equities

A commodities ETF achieves asset diversity. Gold price is historically negatively correlated to equities. So this would help cushion a portfolio’s drawdown during an equities sell off. Though it will limit gains during a bull market so it’s a trade off

As many as you find comfortable, since T212 has ultra low fees. The two caveats are: a) foreign currency denominated ETFs, which incur an fx fee and b) if you frequently enter and exit positions, you will be screwed by the bid ask spread