C&H,
Yes, a case for that many HDDs is a very niche market - generally the server/NAS market. The OP, will just have to get used to the idea of using cages in 5" bays to hold that many HDDs. Some cases are specifically set up to hold hot swap cages for SCSI and now SATA. The hot swap cages are over $100. each by themselves. So you're talking over $500. for a case and hot-swap cages for 20 drives not including power supply(s). Lian Li and Coolermaster sell passive cages that can hold four HDDs each, so you'd need five of them or four plus four integrated HDD bays. Each cage, whether hot-swap or passive, takes up three 5" drive bays. If the 5" bays have the little tab supports between bays, then those will have to be bent back or cut away to accomodate the cages.
This
Lian Li holds 12 HDDs in integrated cages, plus they have seven 5" bays which would allow for two 4-HDD cages plus one optical. That would come to about $300. plus PSUs ( I usually recommend two PSUs for these applications rather than one huge one).
I suppose one fairly normal case that could hold the mobo plus one cage plus four HDDs in its internal HDD rack, one optical etc. and an external case like this:
Codegen 9011 which can hold three 4-HDD passive (or 5-HDD hot-swap) cages plus the extra PSU for them. That would definitely be a low-cost option.
And the YeongYang (yycase.com) yy-0220 cube case looks like it could be made to hold 20 HDDs with a little futzing around. Directron sells those as well as a couple of the resellers linked above.
A visit to
http://www.bowsystem.com would show cases with up to 36 or more 5" bays!!! Plus hot-swap NAS boxes etc. - Quite a smorgasbord of gigundus cases.
I could see one or another of the Stackers being modded to fit 20 HDDs, but it wouldn't be as easy as some of the other options.
.bh.