If you have all your money in mutual funds you don’t need to worry about this question (because your money is managed for you) but if not, here are a few factors you’ll want to consider:
The worst thing you can have happen to you is to buy all sorts of stocks from different companies in different industries and realize that you’ve paid a hefty commission on each purchase you’ve made.
If you want a highly diversified portfolio, the best way to get it is through buying ETFs and index funds, in which case you’ll technically be owning one “stock” but the stock will have funds in tens or hundreds of other stocks.