Technically, you can cram up to four hours* on to a single-sided, single-layer disc (like a DVD recordable), but the quality goes completely down the pan. A single-side, double-layer can carry four hours (four episodes) in very decent quality, so 36 hour-long episodes would take nine discs. 36 half-hours would therefore take five discs.
The problem is more the terrible cases some Studios insist on using for packaging. Here in the UK, Columbia Tristar frequently insist on using an abomination called a Scanavo case which has an inflexible spindle. You can crack a disc trying to wrestle it out of one of those, and a double-sider may be thicker than a standard disc but it's more fragile.
*Note: Eight hours if you compromise the resolution of the picture - before anybody butts in to contradict.
