How to utilise more cores when I am cpu mining?
I don't really know about Litecoin-Qt but for a better result and to configure correctly the numer of threads I recommend you to use a software dedicated to LTC mining.

You can use https://github.com/pooler/cpuminer for only CPU and https://bitcointalk.org/index.php?topic=150331.0 for GPU.

You must have a pool but you can solo mine with the arg '-server' when launchin litecoin-qt.

I know it's more difficult than just use the LiteCoin-Qt miner but it is also more configurable and more efficient.