I'm not sure if DokuWiki really fit's your needs. Note that you shouldn't only consider how many pages there will be but also how many visitors and edits it needs to handle.
FYI: wiki.splitbrain.org currently has about 870 pages - far from what you have in mind. See
http://wiki.splitbrain.org/wiki:experiences:scalability for data by other users. Note that the way how searches are handled changed recently so this isn't the biggest bottleneck anymore.