It's important to be consistent in your writing, so it's always a good idea to decide upon the style you will use and stick to it. Whether you're writing 1000 or 1,000 depends on which style you're following.
If you're writing in numbers with U.S. English, then you would use '1000', meaning one thousand. Otherwise, you would likely use '1,000' in formal or professional writing. In informal writing, such as conversation or in some manuscripts, you may see '1000' used instead of '1,000'.
The important thing to remember is to be consistent with your writing. If you decide to use '1,000', then make sure to always use '1,000'. It's important that your writing is clear and consistent.
