O.K. a PR 6 index page and you want 20 PR 5 pages from it:
There isn't an accurate measure of how much PR it takes to get up a level but I've always assumed it to be logarithmic and use 10 as the multiplier, this fits well with any modeling I have done but you can never be sure.
So, if we assume that a PR 6 page needs 10x more PR than a PR5 then we can assume that you should be able to pass on a PR 5 to at least 10 pages by simply putting only 10 links on the page.
However, there is an additional factor. You can have the case where your index page is a high PR6, with anything up to 99.99 times as much PR as a base level PR5 (or base^2 -.01: base5 would give up to 25 times the base PR5 score), there may be dampening factors but that is a rough approximation of how I see it working.
If your site needs to pass a PR5 to 20 pages then you may have enough with a PR6, I would not be too concerned with the PR being passed anyway as the link structure of the site is more important, make sure this is good for the user to navigate and the benfits will show.
For example, if you have 20 pages you want to rank but they are in 4 specific areas then 4 links from the homepage with good descriptive paragraphs should do better for you than simply 20 links. On the 4 category pages you should have the 5 links to each page with another good bit of text for each, this gives the link some context and thus more weight