How much RAM cache does it need?

[shinkasa4]

cache refers to the option where you can input a size of 16 KiB blocks

I have a few questions:

1. Does deluge use the cache for inactive torrents?
2. How much cache is needed for 2000 total torrents (but only 10 or so will be active at a time) - also the torrents are small, 10MB - 200MB
3. How is the cache used? (short explanation of what the cache is actually used for)

I don't want to set the cache too high because I use the machine for other things.