data / datum / data point
The mass noun, the old singular, and the count form.
The traditional form is plural: The data are clear. That is the form we prefer. The mass-noun singular—The data is clear—is common and is not a mistake. In scientific writing the plural is still the usual choice. Pick one number in a piece and keep it. Datum is rare. For one item, use data point or piece of data.
Examples
Use The data are on the desk.
Use The data is on the desk.
Use one data point
Avoid one data