spectrometric measurements of cell densities are generally done at wavelengths of ~ 600 nm (OD600). However, there is a quite broad margin for the usable wavelengths, so if you use a filter-based microplate reader and don't have a 600 nm filter, pick the one with a transmission closest to 600. Make sure that cells are well suspended; measure the OD right after adding the cells to the plate (or cuvette), as they tend to settle quite fast, which interferes with the measurement.
If you require an accurate cell number, the use of a hemocytometer (e.g. Neubauer-chamber) is generally preferrable over OD measurement, which is rather considered a rough estimation.