The phrase "although countless" is correct and usable in written English.
It can be used to introduce a contrast or exception regarding a large number of something.
Example: "Although countless studies have been conducted, the results remain inconclusive."
Alternatives: "despite numerous" or "even though many".
Although countless paintings have been attributed to him, only a handful with his signature exist.
Although countless commentators speak lazily of the global financial crisis, this is a misnomer.
Although countless variants of poker are described in the literature of the game, they all share certain essential features.
Although countless Greek politicians have been implicated in various corruption scandals in recent years, not one has been tried or convicted.
Although countless biographers and historians have raided the journals, they have never been published in full, and have only been available by appointment at the library in Windsor Castle.
"We've changed our timetable in recognition of the places being destroyed". Although countless ordinary photos have been taken of the sites, the 3D technology the team is using will allow them to potentially recreate the objects as well.
The vaccination programmes of the last century, although saving countless lives, has – paradoxically – left the world's population dangerously exposed.
Although surely countless posts are flagged and removed successfully, the failures are often conspicuous.
Although the countless lush cinematics add a lot to the story, they sometimes get so flaky that you itch for a fast forward button.
Although there are countless people dead and dying in Kathmandu city itself, it is the mountain regions of Nepal that are really suffering.
"Although there are countless reports of the benefits of acupuncture for a range of medical problems, there have been very few well-conducted, randomised controlled trials.
