Actually that depends on number of processor you have.
By default qlik allocates this engines. this is not recommended to change unless you have depth knowledge of the same.
As far as I know windows support only max 10.
In case you need to increase this, then you need to do that by changing desktop heap memory change in a registry.
Have a look at below post for more details.
Error while reloading the QlikView from publisher
But again in any case make sure to take proper backup and snapshot before you do any change in production.
Hope it helps.