Hi Jenn,
It depends on your views on diversification. You have a small sub-set of advisors/investors, that believe you do not need more than 5-10 companies to be fully diversified. Others that believe you need more.
Here are my thoughts. First, what is the purpose of diversifying - it is to reduce unsystematic risk. You can't diversify from systematic risk is risk that affects the entire markets - it is both unpredictable and impossible to completely avoid an cannot be mitigated through diversification.
Unsystematic risk is specific to a company, industry, market, economy, or country. In my opinion, holding 5-10 stocks is not sufficient and you are more exposed to unsystematic risk. Say for example, you hold an equal amount of 5 stocks - all 5 are blue chip and best-in-class. Let's then assume that for whatever reason, one of your companies suffers from a massive event - corruption scandal, fraud, etc - then your portfolio is going to take a pretty big hit. Unfortunately this does happen. See SNC Lavalin, which was once considered best in class and a blue chip stock. Wells Fargo, once considered the top banking stock in the U.S., is still recovering from a scandal it suffered through in 2018. If you held either of these among your 5, you would have significantly unperformed and still not yet fully recovered.
The purpose of diversification is to limit the impact of any one company or industry has on your portfolio. Personally, I fail to see how you can effectively do that with only 5-10 stocks. No company is invincible.
On to the second part of your question - how much is enough. Well, let's let the numbers do the talking. Research has shown that the benefits of diversification top out around 25-27 stocks. Allocated across all sectors, there is proven benefits to diversification up to this point. Once you cross this threshold, the benefits of diversification drop considerably. There are small, incremental benefits but nothing to justify adding more stocks simply for the purpose of diversification.
You also make a good point about keeping on top of your investments. If you don't have the time, then holding a portfolio of 60 stocks can be quite daunting. Not to say a portfolio of 60 is bad, however it does little for the purpose of diversification.
Hope this helps,
Mat