DOS 2.0 suports a max of 720 128 or 256 byte sectors, so ~90K or ~180K per disk.
DOS 2.5 Added ability to support up to 1024 128 byte sectors (Leaving 16 or so unaddressable by DOS) for ~128K per disk.
Maybe DOS 2.5 could technically adddress 1024 256 byte sectors for 256KB per RAMDISK?
DOS 2.0/2.5 can be setup for any combination of drives 1-8, details of which I posted about previously: http://atariage.com/...e-d2/?p=4132426
Drives 1,2 5-8 = 11110011 = 207
DOS 2.0 default of Drive 1,2 will show 3 = 00000011
DOS 2.5 Drive 1,2,8 may show 131 with PEEK(1802) = 10000011 in binary? I have not actually checked this - The 2.5 manual says default is 3, so maybe RAM Disks do not need to be included here.
Poke the new value into 1802, write DOS files to disk, and reboot.
The DOS 2.5 manual does indicate, however, that the practical limit is 5 drives enabled, since each enabled drive # (including RAM Disk) will raise MEMLO, and beyond that it will conflict with the loading address of the DUP menu. If you don't use DUP, I guess you have more flexibility.
EDIT: I just booted a DOS 2.5 disk, and can confirm that ?PEEK(1802) returns 131, for D1,D2,D8