So damn hard deciding which tube preamp to buy!

facten, that may well be true, but you should give him the benefit of the doubt. I wish some one would of made a few suggestions on a tube preamp for myself but in the end I had to experiment with a hand full of them and decide which one worked well to my ears and in my system. That is what happens in the end any way.

Why do some tube preamps have lots of tubes, arc has 8, bat has about 15 for rex, vac has only 2, vlt has 2.  Are they hybrid?

they seem to be located in gain stage.  Why here?

This article explains it pretty well.  
http://www.moxtone.com/Tube_buffer_en.html

BAT uses tubes in the rectification (power supply) vs a SS rectifier.  This is technically a quieter approach.  My experience is that in practice, this is only really critical with highly sensitive speakers.

They then use tubes in the gain/amplifier stage and the buffer.  Alternatively, Conrad Johnson uses two 6922s in the amplifier stage and then MOSFETs in the buffer so yes, this is a hybrid approach.  
Many companies take very different approaches and each designer often has very firm opinions on what does and doesn’t make a good tube circuit.  
I have had one designer tell me a 6922 is not appropriate for audio applications.  I have other designers where it is their preferred tube.  

The important thing is picking a preamp that will marry well with your power amp.  

CAT Renaissance BPX with phono. IME nothing else even comes close.
You can also order it with Balanced inputs/outputs if you so desire.

ANK Mentor would be a dream preamp for me.