Im looking for a usb interface for recording some jam sessions. So I want a usb interface where I can put 8 Microphones (for drums, guitars, bass and voice). My question is do I need a preamp for each microphone, because Interfaces like the Focusrite Scarlett 18i8 2nd Gen say, they have 8 analog inputs, but only 4 preamps and with additional equipment its possible to have 18 channels? I thought about buying the Behringer UMC1820 U-PHORIA, it has 8 preamps, but do I need them to record 8 Microphones? It also says 18 ins / 20 outs? Im kinda confused about what is necessary and what do I need to get a decent sound. I guess you can split up preamps or something like that, but your gonna lose some quality?
In/out counts are usually heavily inflated and count S/PDIF, internal mixes, headphones and whatnot. They are more or less what is available digitally, and even then you might not get everything at once (like having to decide between ADAT and S/PDIF inputs).
You need a preamp for every mic. There are boxes with 8 preamps from several manufacturers: basically the price range is something like €150 to €2000 for them. Of course, there are always more expensive options.
"Recording some jam sessions" with 8 mics suggests thinking about pitching for a mixer with digital output instead which you'll need for PA purposes anyway (well, the mixer, not necessarily the digital output). Note that a mixer is quite more bulky than a mere recording interface. You can either get an analog mixer with recording outputs in which case you'll still need something taking 8 analog inputs, or one with digital interface built-in.
Note that you are already at 8 mics and you should leave room for extensions. Mixers with more than 8 inputs are particularly bulky. For drums, it is not uncommon to use a separate submixer.
At any rate, the most important ingredient to "get a decent sound" is a sound engineer. A band member setting up mics and afterwards trying to figure out how to make something nice from it is going to be hit and miss at best. Particularly if he still has to learn his sound engineering chops.
Having someone running around and adjusting and trying things during the rehearsal is going to lead to likeable results a lot faster.