'countless thoughts' is a correct and usable phrase in written English.
You can use it when you want to express frustration or confusion, or when there are too many thoughts to count and you want to emphasize the large number. Example: After the argument, I was left with countless thoughts running through my mind.
He's collected countless thought-provoking notes, and is now publishing them in a book which he expects to be out later this month.
Countless such incidents, thoughts and initiatives have formed me as a person.
The painfully extended bit about answering the question "What does a yellow light mean?" still cracks me up, and I've seen it countless times: Your thoughts?
Spontaneous phone calls, emails and instant messages interrupt our train of thought countless times each day, ultimately affecting the quality of our work.
The temporal-parts ontology implies that each of us shares our current thoughts with countless beings that diverge from one another in the past or future.
In the November issue of House Beautiful, we get a look inside his beloved Monticello, the home that not only houses hundreds of antiques but also a countless number of design thoughts and ideas from our third president.
One evening about two months ago, Rena Effendi, 35, a photographer, entered a cab with a licensed driver in one of Cairo's wealthiest neighborhoods — as she had done countless times without a thought before.
Pains and visual images, as well as countless other kinds of thought, possess a kind of subjectivity that cannot be captured in a purely scientific account, because scientific descriptions concern only the objective properties of natural phenomena.
He's also given himself steampunk X-Men powers and thought of countless ways to launch hundreds of fireworks into the sky over his Stamford, Lincolnshire suburb.
