A couple of things:
1) you always need USBD.SYS for ALL devices, no matter what device class. USBD.SYS is the "mediator" that interconnects the client drivers for the various device classes (USBHID.SYS, USBMSD.ADD, USBPRT.SYS, USBCOM.SYS, USBAUDIO.SYS, USBETHR.OS2 etc.) with the host controller drivers that do the "real data movement" work (USBUHCD.SYS, USBOHCD.SYS, USBEHCD.SYS)
USBHID.SYS has the special job that it is serving the "HID" device class which encompasses any USB device the user directly interacts with and by itself is the "mediator" for USBKBD.SYS and USBMOUSE.SYS which serve specific HID device types (keyboard and mouse respectively).
2) use HCIMONIT.EXE to tell you what host controllers you have and what type they are (UHCI, OHCI or EHCI). Then, supposing you have 5 UHCI controllers and 1 EHCI controller you need this as a minimum in config.sys to operate up to 4 USB mass memory devices:
BASEDEV=USBD.SYS
BASEDEV=USBUHCD.SYS
BASEDEV=USBUHCD.SYS
BASEDEV=USBUHCD.SYS
BASEDEV=USBUHCD.SYS
BASEDEV=USBUHCD.SYS
BASEDEV=USBEHCD.SYS
BASEDEV=USBMSD.ADD /REMOVABLES:4
And as ivan has correctly pointed out you of course need to install the file system drivers for all file systems you use.