The general rule is 1 per page. If you were going after "red widgets" make a page for red widgets add content for red widgets but please do not talk about blue widgets. If you do this Google will know that you talk about blue and red widgets but not which one you want to rank for. Additionally, try to have different pages for terms like "red widgets" and "red widget reviews" these two terms are very similar but they should be categories or separate pages from each other instead of on the same page.
