Re: Number of root folders in the document library of a site
No, you can't.
I would recommend a folder distribution with no more 1000 folder / document / nodes per level. And this as a maximum, it is not necessary to go until the maximum. Many database queries will suffer when applications or Share have to deal with big subsets. And this may have a big impact in performance as cited above.
Tipical structures are based in volumes (let's say subfolders with no more than 1000 folders each), or structures yyyy/MM/dd in the same that Alfresco uses in contentstore to distribute files in an homogeneus way.
Think in a disk with 65k documents in a folder, you probably have problems opening the folder, listing files, generating previews..