This presents a curious dilemma for investors at any level of expertise. Follow just a handful of markets, and you’ll have a much better, deeper understanding of them, but you may find that your opportunities for investments that meet your criterion are quite limited, which may cause you to begin trading more aggressively than is wise, which can lead to catastrophic losses.
On the other hand, watching a number of markets opens up the field of possibilities and can help control your aggressive trading impulses, but the more markets you watch, the more your deep knowledge about each one is diluted, which will also increase your risk.
How many markets you follow is a question only you can answer.