Welcome to the forum!
I'm not clear what you're asking. Is the image an imported raster image consisting of 20 black rectangles? Or are they vector rectangles?
If they are vector rectangles, they are already selectable. There are several ways to make a selection in Inkscape. One is to drag the Selection tool
around all of them. Or you could hold the Shift key while you click on each one, one at a time. Or there are some other less known ways. If there are a lot of other contents in the file, you could use Edit menu > Find, to select only the black rectangles. (And still other ways too!)
If you want to create more rectangles, you could select all of those 20, and simply Edit menu > Duplicate (or use the button
or use the key shortcut Ctrl + d). If you don't want 20 more, you can select however many you need and duplicate them. Note that when you use duplicate, the new ones are created exactly on top of the originals. So you'll have to drag them away. If you drag them away immediately after duplicating, you can move them all at once, and not have to do it one at a time.
Or you might want to clone them, if you're thinking of many more rectangles....although if you clone them, they can only be one color, they can't have different colors. Hhmm, well that's not entirely true. If you use Edit menu > Clones > Tiled Clones, you can set them up to have different colors. But that might be a little more advanced than what you need, right now, since I'm not positive what you need yet.
If the image with black rectangles is a raster image, you won't be able to select them at all. But you could use Path menu > Trace Bitmap, to make them into vector objects.
Or you could forget about using the vector image at all, and just draw rectangles using the Rectangle tool. For example, draw one rectangle. Duplicate it. Select the first rectangle and the 2nd one, and duplicate those. Now you have 4. Duplicate those, you have 8, then 16, then 4 more, and you have 20.
