I don't know, it depends on so many factors. But the image size is usually 1 micrometer x 1 micrometer to 10 micrometers x 10 micrometers. If you want to see, say, 100 particles per images, you should be able to calculate an approximate concentration and volume to use. Typically you can fit 3 to 30 microlitres of aqueous solution on a small 8mmx8mm sample.