There are plenty of synonyms that can be used in place of the phrase "ten thousand". Some of the most common synonyms include myriad, countless, umpteen, innumerable, and an infinite number. Other options include a great many, a horde, a legion, a multitude, and an army. In a more formal writing or speech, the word myriad can be used to replace ten thousand. It's important to choose the right synonym to replace a specific word depending on the tone and context of the writing. With these synonyms, writers can add variety to their vocabulary and make their writing more interesting to read.