Wormholes

Depends on how often you run into the natives.

When I first started out, in late 2009, I solo'd a C4 with a small tower. Hardly feasible anymore.

Also, depends on your statics. A C3 with a static low, for example, has more opportunity for pews than one with a static high and can keep more people interested. A C4 might have less traffic but more interesting systems to connect into.

I'd say:
C1: One or two newbie players, or more if they just want to check out wh life, but they'll be bored soon and move on
C2: Viable for one or few players, but depending on statics also for medium-sized corps. If the hole offers nothing interesting some days, people can just go into hisec/losec and do something else, e.g. scan for other wormholes from there. Most versatile class of holes
C3: Made for solo farmers. Best ISK/hour for one player with single Tengu or dualboxing. Corps I've been in collected a lot of Tengu kills from C3s. Useless for larger groups or players looking for serious pvp
C4: Niche class for older players who don't want to or can't join larger entities. Deep space, but valid for small groups or even solo dualboxers. Only used as static hunting ground by everyone else
C5&C6: Suitable for larger groups, if you survive in here for long with a small corp you are just lucky.

C1&C3 are shallow wormhole space because they don't have statics to UNknown space. C2 is the bridge because they have statics to both w-space und k-space. C4 is isolated deep space. C5&C6 is really deep space but can be connected to k-space and is generally less isolated.

c4 with static c3s. Best thing ever because your POS is generally isolated and if it is a properly fit large one, difficult to take down in a c4 hole. Also the C3s provide the best income and occasional gank and a neverending supply of anoms.
Thirdly, if you have a small corp you can do the c4s with a group of 3-4 tengus and you can duo the 3s with 2. But beware c4s anoms spawn painfully slowly youll be making most isk out of your static.

Goodluck

I have heard that a small group of russians lived safely in a C6...

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Oh wait

Just go and live in a c5 you can easy survie with 3-4 people in a c5 just be sure to setup a faction tower with faction mods on it so it looks soo scary nobody will ever atc you.

It really depends on a couple of things:

1) How active people are. If the corp runs the sites 1-2 times a week even a C1 can have 5-6 semi active player. (Generally in this case the worm hole is for people's alts to make isk for pvp.) You'll make good isk not great isk. It's low risk a POS, hardeners (pos), some ecm (pos), and 1/2 dozen drakes. This will work up to a C3 (other than the radars and the like.).

2) How many things do your player want to do. Beyond running the sites there is also rock and gas mining as well as PI. These can be lucrative between the rarer gas, and the abc ores.

3) What your statics are. A C1 with a static C3 will get you many more opportunities to run C3 sites than a C3 with a HS static. Once you understand how to effectively collapse your static it's pretty easy to chain collapse until you get suitable hole to exploit. Th downside is that you generally won't have a static HS. C2 are nice as they have 2 statics (or at least some do). In the past I've made a lot of isk farming a C2's static C1, while enjoying easy HS access. On the downside C2 with good statics, and mix of planets are generally claimed.

The thing to remember about a wormhole is that you can't depend on farming sleeper sites, and mining any class of wormhole to keep you busy. Sites just don't spawn that quickly, and you can have dry spells of week or 2 when almost nothing . Either your corp needs to have other things to do or you need to exploit a static connection to harvest other systems.

Anywhere between 3-100 people.