Spread & risk

Spreading (diversification) reduces so-called 'unsystematic risk' — the risk of individual companies or sectors. What can't be diversified away is 'market risk': when the whole market falls, almost everything drops with it. Rule of thumb: from ~20–30 well-chosen stocks across sectors, the diversification effect is largely exhausted. With one broad ETF (e.g. MSCI World) you get it in a single position.