I'd normally select the files manually, using the status bar to see the selection count and stop at the right number.
Alternatively, if you turn on the Index column (from the General category), you can type # into the file display (the Find-As-You-Type field will appear in Range mode) and then select ranges by their index numbers. For example, #1-10 would select the first 10 files.
Alternatively, the SelectEx script add-in gives you a command which can select a specified number of files, or every nth file, and so on: