We've always said that we would hold six to eight stocks, but six is the more likely number. This means that we don't need to work as hard to find our main big winner. Hopefully we have a handful of steady performers. On the other hand, buying more stocks would diversify our holdings further and thereby dilute the risk that we face in each individual stock. Note that adding more companies to a portfolio is a way of diversifying risks, not avoiding risks.
New stocks bring new risks.
All that said, I believe that only if the opportunities presented by new purchases are truly recognized as greater than the opportunities offered by current holdings does investing in additional companies make sense. That, or we should only buy more companies if we're concerned about our current exposure to risk and we want to dilute that risk and accept the added costs and new risks of doing so. Overall, we seek simplicity. In our case, owning a small handful of companies that we truly understand, even only four companies if it comes to that, should do the trick. Many people do.