Does the Pagerank number by itself will have a tendency to drop over time due to a general growth in the web...even when SERPs remains the same? expert comments expected
Yes. If you look at the original PR documents, PR for the entire www is a finite amount. So as the web expands, the amount available for each page drops a minuscule amount. This means that over time, there is a natural tendency for a pages PR to decrease (but could easily be offset by adding more links to the page!)
Page rank effect SERP. Mine was 0 in december and a few hits from google in a day. But now its 3 and 2500 hits from google. So PR affects your SERPs. But both are not same. SERPs may drop if you dont update your contents
PR certainly spreads across other pages as their number goes up. But you can limit that by making your links nofollow. So that PR stays on the pages where you have been able to generate inbound links to...
The amount of PR grows as the web expands because each new webpage brings in a tiny bit more of PR value, which can then be passed to other pages. I'd guess that PR can grow above 10 (if you have enough backlinks that is), but there are very few pages that have PR 9, even fewer have PR 10, so try to imagine how many backlinks it would take for PR 11, 12, 13 (but again said, this would ony be a guess that you can grow above PR 10). SERPS on the other hand have to do with keywords and keyphrases, although PR does play a role here it is only as far as you keywords are concerned.