The phrase "countless number of people" is correct and usable in written English. It is often used to refer to an unspecified large quantity of people. For example: "At the protest, there were countless numbers of people rallying for change.".
It would affect countless number of people who will find it difficult, if not impossible, to pay back the bonuses that they earned".
I've had an amazing Olympic sailing career and I would like to take this opportunity to thank the countless number of people who have been involved in my sailing career to date.
After the explosion on April 20, there were some cancellations, but what really wrecked the summer for TradeWinds was the countless number of people who feared that oil was about to hit St. Pete and never called in the first place.
Plus, the Crumpler logo is pretty well known out here the countless number of people I've passed on the streets in Barcelona, I've had one recognize it.
Countless numbers of people move to a higher living standard.
I knew countless numbers of people were praying for me.
As a psychiatrist for almost three decades, I have tried to help countless numbers of people change.
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.