one filter 

It came with the 10 stop glass filter and seems to be pretty great quality. I'm loving using it. Even ordered a 6 stop to go with it. 

No, you need to thread the holder onto the lens itself. 

Hello, this is a high end filter kit. Of course these filters are made of optical glass. Regards. 

your lens curvature will require an adapter for any filter system. I saw the Lee filter adapter for this lens...but it seems it does not fit all filter holders. Lee filters will work with the adapter. 

Hi，this is a square lens filter kit , has 8 adaper rings can fit the lens size from49mm-82mm, and the filter is square , 100*150mm/100*100, if you filter size is same , it would be ok ,thanks. 

No. This filter holder can only hold 2 filters. I believe the more prudent course would be to get a filter that is rated for more stops and can replace two filters that are rated for less stops. There are filter holders that do hold 3 filters but they are smaller than the K&F filter holder (which is 100 x 100 mm). And in my opinion, the quality of those 3 fitter holders that are located on Amazon is less than the K&F system. 

Yes, you can add filters in this holder, two to be exact and as far as the thickness of the filter goes, I have no idea. I own several different name brands of filters which I use in this holder without any problem. All the filters I own fit nicely into the holder and for myself, I have never really given any thought to the filter thickness, however you will be able to fit only one filter per slot. Sorry I cannot be of more help but this filter works great for my photographic needs.