The phrase "mere thousands" refers to a small number of something, typically less than ten thousand. It suggests that the quantity being referred to is not considered substantial or large. Full definition
Studies of clay extracted from the seabed, supported by new theoretical ideas, suggested that ocean current patterns might shift within mere thousands of years. (history.aip.org)
If you bear in mind the fact that the human race has been in existence not for mere thousands but for hundreds of thousands of years, and then think of the life of mankind on earth up to date in terms of the life of a single human being, you will see that 37 A.D. is no farther off from 1937 A.D. than yesterday is from today in your life or in mine. (religion-online.org)
For more than two centuries, careful scientific research, much of it done by Christians, has demonstrated clearly that the earth is billions years old, not mere thousands, as many creationists argue. (religion.blogs.cnn.com)