Do you know of any plugin that best fills the page with as much copies of an object as it can? I don't care about orientation (they can be vertical and/or horizontal as well), and it would be a plus to specify in any way a minimum spacing between copies, for cutting purposes.
Some time ago I asked how to create Symbols in Illustrator in order to generate a mosaic. I'm not really sure that "mosaic" is the word - I need to fill a whole page with the same little image, so I print lots of copies in a single sheet of paper.
I've tried using the approach of Transform's copies in the last answer, but I was thinking there has to be a plugin for doing this - come on, I can't be the only one in the world printing labels.
Any alternative method that doesn't involve loosing quality of the image can be valid, but sticking to Illustrator would be great.
Although not a plugin, this is an ideal case for a Pattern in Illustrator. In your case it can be used to generate many copies of an object to be placed on a single page.
From your description it sounds like you could be talking about making an image into a pattern. In Illustrator, it is actually possible to create patterns out of images. You'll want to select the entire object you want to repeat and then select Object → Pattern → Make...
The dialog box will prompt you in how you want to arrange your pattern and after you select Done it will save your pattern as a swatch. Below is actually a jpeg image created from a vector and then used as a pattern.
You can then apply this swatch to any shape, but in your case it makes the most sense to create a rectangle the length and width of your printable area and apply the swatch fill to that shape. The slight downside is that it might be difficult to align the pattern to perfectly fit your printable area, but that is accomplished with some tampering in the pattern dialog box.
Edit: Looking back on this answer it is kind of a janky way to set up a sheet to print labels as you stated in your post. I couldn't find any plugins that did this for Adobe Illustrator and I think this is because Adobe Photoshop has File → Automate → Contact Sheet II... which takes care of this quite well.