I'd use a right-align tab. If you are doing this in a paragraph style, it's pretty easy - set up an extra right-align tab in the Indent and Tabs sub-panel, and then add an extra ^t (which represents a tab) before the number placeholder in the Bullets and Numbering sub-panel, in the "Number" field in the "Numbering Style" section.
If you're not using a paragraph style, then, well, you should be. I know there is probably a way to do this if you're just going to Type -> Bullets and Numbering, but I couldn't tell you what it would be.
I don't know if this is what you want to achieve, but take a look at the screenshot below and see how to right-align numbers in autonumbered paragraphs. The secret is to set the "Bullet or Number Position" alignment to "Right" (highlighted in the screenshot).
Adjust the "Left indent" and "First Line Indent" according to your needs. In the image, the guides show where these fields apply.
Hope it helps.
Thanks Joel. I am using paragraph styles, and had been fiddling with the Bullets and Numbering dialog box options. Got to right align, but it didn't work until I came across a David Blatner post that mentioned the left indent had to be corrected. So I am good to go. I like your idea of the extra ^t in that subpanel too. Thank you!
Thanks JoaoCP, I appreciate your post!