There is a limit on the number of locks that can be placed on a file. If you are accessing a dBASE file from a server, the limit depends on the server (refer to your server documentation).
If you are accessing a dBASE file locally, the limit depends on the buffer space allocated when SHARE.EXE was loaded (refer to your DOS documentation). If you are exceeding the number of locks available, you may want to switch to file locking.