do sega cd games need a memory card

AFAIK all SCD units have a small amount of internal memory - the CDX being no exception. Don't expect it to hold all the save games from your RPGs tho' - it's not very big.
so there are no mem cards at all for the sega cdx?, will the system memory hold 5 game saves or more or less, does the system memory run on a battery like the saturns and would i need to replace it ever?
There is a Sega CD backup cart, called a RAM Cart for some reason. It looks like a Genesis cart but with blue stripes on it.

Game saves vary on size, depending on the type of game.
The internal memory works on an internal rechargable battery, which should last many years. Mine still seems to be going strong.