I second Burke's comments - ideally you would move this data into a mySQL database and migrate to a *real* database.
Of course, this may or may not be welcome news. I believe your problem is that Windows is attempting to do 'byte locking' on your database file - that is, someone opens the file and requests some data. The part of the file that contains that data is locked, and the rest left open for other people doing different operations on other parts of the data.
The number of users it can support is generally proportional to the size of the data - if one person is working on record X, no-one else can work on it regardless of anything else. But they can work on record Y if no-one else is using it. The 10 concurrent users is a limit of NT4 Workstation which has a max number of users set at 10.
Here's a link to the samba documentation which goes into it a bit, but doesn't really help.
http://de.samba.org/samba/ftp/docs/h...on.html#AEN199
All in all, I don't think there's much you can do. I don't think Samba handles it very well, and your best bet is to either :
1) move the file to a Windows share.
2) move the data into a mySQL database (which is actually altogether a *much* better option. But conversely alot more work).
Slick.