Well, there may be several files associated with each download: the xxx.part file, which contains the partially downloaded file's data; the xxx.part.met file, which keeps track of information about the file being downloaded, the xxx.part.met.bak, which is the backup of xxx.part.met; xxx.part.met.seeds, which keeps a few sources for rare downloads between aMule sessions.
However, only the xxx.part files are kept open all the time. The other files are only open momentarily when they are read or written.
Remember, though, that open sockets (network connections) also count against your limit of open files. So, if the number of xxx.part files plus the number of connections approaches the limit, then you will start to have problems.