The more you have the more it will use.I run Lightroom using 32Gb ram checking activity monitor I have seen it hit 19gb at times. Really not sure if this is normal or not, but it is quite ram hungry I believe.
Thanks for your inputI run LR on 8GB and don't have any issues with memory. Usually I have a bunch of Safari tabs and iTunes open as well.
If I was buying new today I think I'd go with 16GB. But I've had no issues at all with 8GB and I'm sure you'd be fine with it.
The more you have the more it will use.