There is no direct way of doing this in Dewesoft, but it can be done with the use of the batch file. 

set "NumberOfBackups=6<- number of files the customer would like to keep in the folder
cd %1 <- this is folder location parameter and can be set in sequencer
for /F "skip=%NumberOfBackups% eol=| delims=" %%I in ('dir "*.dxd" /A-D-H /B /O-D 2^>nul') do del "%%I"

Use the file manager block to open the file and put the location of the folder in the parameters text box.