You can have as many plastic figures as are available. So for most nations, you can have 6 army figures (which is all that are available in each color) while the Russians can have 7 because they get the extra white one.
Your friend may have been confused perhaps by the stacking limit, which is how many figures you are allowed to have in one space at a time. This normally starts at 2 for most nations, but I believe the Russians have a stacking limit of 3. Certain technologies will raise this limit.
Just clarifying that by "one space" he means one square, not one title(the flipped map card).