With the global demand for vaccination and reliability of Pfizer vaccines, there is good potential for further growth!

You should not have a personal mandate on how many stocks you hold. You will restrict what you can do.
Imagine you tell yourself to only hold 10 stocks. One day another stock suddenly become a great bargain what will you do? Liquidate an existing stock just to add this bargain? When that liquidated stock could be a bargain as well.
Instead, another way is you could add this great bargain too to your portfolio (if you have spare cash). Then have a plan after that to trim holdings maybe? Like if it run up 50% with no apparent reason can consider to sell and add more to your current holdings.
The thing is, don't be jealous about those people who all in on one stock and it become a multi-bagger. There are a lot of people who did the same but see their holdings get wiped out. Most are no longer on this forum also anyway since once you wipe out your money you are unlikely to continue investing anymore.
If you throw all your money into one stock but lose sleep then its not worth also. Definitely if you diversify more then your overall portfolio will only be at best growing slowly since if one stock suddenly run up your overall portfolio gets pulled back by the laggards. But at least its steady?
Always remember to DYODD....