Hi,
I have a question about the search refiners avaiable in sp 2013. One of the options to set is the mentioned above maxNumberRefinementOptions (I'm setting it through powershell so no spaces). Where can I find any documentation about this setting? I was wondering e.g. what is the max value of this property.
Thanks.

By default it's set to 20 (i believe). It's a performance question, adding more refiners increases the load on the search server and slows responses. Whilst you could increase it i would avoid doing so if possible, in most cases you can avoid having to approach that limit through better design of your refinement options.

If you do decide to increase it I would run some tests to see what the impact is. Run a search for a term with 10 refiners with SharePoint dashboard enabled. Do that search 10 times and then record the average time taken. Add another 10 refiners and repeat the test. Then try increasing the limit, upping the total and repeating a third time. That'll let you see what impact going beyond the recommended limits are.