I finished my model and if I render it using 128 or 2048 samples it looks the same.How many samples should I use? The only thing that samples do is to rid of the grain?
You find the right amount of samples by testing what is enough to get a clear image
Samples represent the calculations of how each pixel should look. During one sample a path from a pixel to a light source is calculated. A ray travelling that path is effected by various interactions with surfaces in the scene and this is how the colour of the pixel is determined. The same pixel can be a result of light coming from a lot of directions so the more samples, the more accurate the result, because they each calculate another path checking if it goes to some light source.
So simple scenes with direct lighting need a lot less samples than complex scenes with a lot of indirect light or caustics for the render to have less noise/grain. To determine the correct amount of samples for your scene you should test, how many samples is enough. You can start with fewer samples and make test renders to see if it's enough while increasing the amount. If you do this with the whole image this might take too much time so you can render only regions for testing (ctrl+b in edit mode to define region, ctrl+alt+b to remove region):
It is best to chose the regions that are the darkest lit places that will have the most indirect light and therefore are most likely to contain noise or any areas that you suspect might have more noise. You can also choose progressive rendering and just look at the render while it's going to see how many samples is enough.
You should use how ever many samples looks good.
But are you talking about Cycles? Evee? Workbench? The renderer you use makes the sample number mean a different thing. Cycles also has two methods of generating samples (branched vs non-branched).
The complexity of the lighting in your scene will also impact how many samples you need.
It's actually a pretty complicated topic that depends on what renderer you are using. Changing the number of samples effects multiple different passes, not just the obvious anti-aliasing pass.