There is no answer to this question.
You will have to find out the hard way.
There is probably a limit somewhere, beyond what one's computer can handle, but I do not recall seeing that.
One limitation is the 24 hr. Duration on a single Sequence's Timeline, but multiple Sequences get around that.
In earlier, I have done a Project that ended up being just short of 4000 SD-sized stills (not the same, obviously as Clips), several 100 MOV 3D animations, almost as many DV-AVI's, 100's of Titles, and 12 Audio Tracks, going to a 5.1 Master. Not one problem. These Assets were also on a FW-800 external. Yes, it took about 4 mins. to load, but no issues.
The connection of your externals could present problems, but with FW-800, or eSATA, there should be none. Working from FW-400 was too slow for me, and USB 2.0 connections came with their own set of issues.
Now, I was editing to/from, so it was more intense, than just accessing Assets. Hence, YMMV.
Maybe Todd, or Jeremy will know what the real "ceiling" is, but do not think you will be near that - whatever it is.
Thanks for the answer. I think what I'm looking for is some other editor's real life experience, much like you
There might be someone who has had a project that held 600 clips or something like that and I can extrapolate from that.
There have been comments on Clip numbers up into the 1000's, but do not recall anyone hitting some ceiling.
It seems that the first stumbling block is with computer resources, regarding # of Tracks, and the types of material on those.
Also, and something that I might not have specified, my experience with that "opus" was all in SD, and not HD, like AVCHD. My older workstation never even hiccuped, but the material was pretty benign, and also sized for the Project. Only "oddity" that I had were those MOV files with the Animation CODEC, and they were smooth as silk on my PC.
Good luck, and maybe some of the heavy-hitters can share their Projects' specs. with you.
From somebody who acutally uses CS5 i can tell you that i have had no trouble loading up over 10 hours of hdv.
It all depends on the footage and the horsepower you are using.
I do long form HD projects in CS5 with well over 1000 HD clips, music, narration, graphics, etc.
There is no difficulty with PPro handling it all.
As Ann implied- if there are going to be problems, they will most likely be due to lack of horse power of the system rather than any inherent limitation of PPro.
I am waiting for Todd and Jeremy to return from their holidays with their families, so that one can give use the true "ceiling."
I anticipate that it will be like the HDD usage of Scratch Disks in Photoshop, as of CS - something like 32EB, which even Harm has not reached yet.