According to this Wikipedia article, you are allowed 3,000 files per app but I was reading a thread on Google Groups that someone's Java app received a warning when it tried to upload more than 1,000 files - he got around it by bundling some files inside jars. Which is correct?
Updated from [Python Appengine Quotas and Limits]
Guido van Rossum provides the answer in Google App Engine issue 161:
All, the combined limit on static and code files has indeed increased to 3000. There is no plan to increase it further. The following limits are also still in place:
150 MB max combined size of code files
10 MB max individual size of any file
1000 files max per directory (not counting files in subdirectories)
In the quoted message, "blob" refers to static files; "file" refers to code files.