What are the most commonly used filters for SLR cameras that mainly shoot people?

It depends on what kind of portrait is taken.

If you shoot the interior in the shed, you generally don't need to use a filter, but if you shoot the exterior, you can get better results with a filter in some occasions.

Filters are generally used for landscape photography.

The filters we usually use are: uv mirror, polarizer, gradient mirror, medium gray subtracter, color filter and so on.

UV mirror can filter out the ultraviolet rays in the light and make the picture more pure.

At noon or on sunny days, you can use a medium gray reducing mirror to moderately narrow the aperture and extend the shutter time to obtain more gorgeous colors.

When shooting at the seaside or in front of reflective objects, such as glass windows, you can use polarizers to reduce the reflection on the water surface and increase the transparency of the picture.

When shooting outdoor portraits with the sky as the background, gradient lenses can be used to reduce the brightness of the sky, so that the brightness of the picture can be balanced up and down, so as to obtain more picture performance effects.

In order to obtain a special hue, color filters with different color temperatures can be used to make the picture look colder or warmer.

Generally speaking, there are too many filters for landscape photography, so you can choose according to your own needs when taking portraits.

I hope my answer can help you.