From the DOSBox readme.
IMGMOUNT DRIVE [imagefile1 imagefile2 .. imagefileN] -t cdrom -fs iso
imagefile1 imagefile2 .. imagefileN
Location of the image files to mount in DOSBox. Specifying a number
of image fil […]
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imagefile1 imagefile2 .. imagefileN
Location of the image files to mount in DOSBox. Specifying a number
of image files is only allowed for CD-ROM images.
The CD's can be swapped with CTRL-F4 at any time.
This is required for games which use multiple CD-ROMs and require the CD
to be switched during the gameplay at some point.
Say you have the 4 CDs imaged as ISO files and place them in C:\DOS
The mount command for this would be
IMGMOUNT D C:\DOS\ROTH-1.iso C:\DOS\ROTH-2.iso C:\DOS\ROTH-3.iso C:\DOS\ROTH-4.iso -t cdrom -fs iso
To swap CDs press Ctrl+F4.
I think Bladeforce assumed you know how to image a CD. For that you can use a program like MagicISO, ImgBurn, or ISO Buster.