Well, I´ll see what I can contribute to help a fellow writer ^^
Your question are direct, so it shouldn´t be a problem giving my view on it:
Travelling through Kanto getting gym badges?:
Well, you have to consider foremost that not all trainers decide that they want to join the league and thus battle for gym badges, even just a normal journey with an occacional battle can be satisfying for some trainers.
Ofcourse, 99% of the readers likely want awesome battles, so instead of Gym badges, you can have trainers just battling for the sake of battling. Having a goal is important, but doesnt always have to be the usual ´´ get all badges so I can join the league ´´.
While I yet do not know your story or characters, I´d say give some thought if gym battles and leagues really fit the characters you had in mind, since as I mentioned, not every character needs it and with different goals, it makes characters much more interesting.
6 pokemon or more caught?:
It´s pretty standard I think that most trainers end up perhaps not immediatly reaching up to 6 Pokemon caught, and if they do catch more, then comes the dilemma of giving all Pokemon fair time in the spotlight, unless ofcourse you plan to focus just on the 6.
Could ofcourse do like Ash did in Frontier, after he returned from Hoenn he left some Pokemon behind while keeping others, and also returning old ones like Charizard. So having more than 6 gives you the possibility to vary Pokemon battles alot more.
But then comes a problem, that if some Pokemon are really uninteresting, it´ll lower the interest of readers, and definetly slows down pace and development, not only in the Pokemon but also trainers, since maybe you have to spend many chapters to make them interesting.
Pokemon fully evolved or none evolved or just specific one's :
Ah, this is always a no brainer. Decide based on the Pokemon itself, its personality, the story and ofcourse the trainer´s need. By default, any trainer wants their Pokemon to fully evolve, but as always, they are exceptions which in time turned out to work well.
I for once can´t imagine ( okay maybe alittle, but not much ) Ash´s Bulbasaur as a Venusaur, Pikachu as Raichu, and so on, because all three are already so interesting, that while evolution would be interesting, in their case maybe it isn´t necessary at all.
But ofcourse, it doesnt able to all Pokemon, so while it work´s for Ash´s Pikachu, maybe another trainers Pikachu would benefit from an evoltion. Also, evolution makes things very interesting since the Pokemon team dramatically changes in terms of new changes.
For instance, a trainer commanding a swarm of Beedrill ? Sounds pretty interesting ^^! A trainer commanding a group of Caterpie ? Hmm perhaps not as much... also, personal preference on your end plays a big part in what Pokemon you want the trainers to have.
People on journey = 1,2,3,4,5 or more?:
Hehe, if I´d be a trainer, I´d have the whole town joining me and rooting and cheering and, okay Im not Gary XD. But in all seriousness, don´t see it as how many trainers, but what trainers to have on the journey. Does your protagonist really need two redhead girls ?
Add them ^^! If in need of an other kind of teammate, focus on that then. The protagonist can define the team, just as much as the team can reflect the protagonist.
Sadly in Ash´s case, that ofcourse wasn´t too much evolved into, so in your story,if characters fit, make sure it is not only storywise but also personality and maybe goals.
As reference, in my current fanfiction project ( Pokemon Fusion ), Ash adds Misty to return to his brand new team not only because she was become MUCH stronger, thus able to help him train his Pokemon and such, but also because they get an emotional connection.
Ash also adds Gary, because they have been rivals for so long that Ash never saw him as a ´´friend´´. But in time that changed, and with Gary alone and free, Ash gives him the offer to join the team and for them to start being more friends than rivals all the time.
Last, Ash adds Tracey, not only since he ofcourse has a good bond with Misty after she stayed behind, but because Ash felt bad for leaving him behind, and having had so much fun in the Orange Islands before, Ash takes the chance to get Tracey back on the road.
As you see, I don´t add characters just to add, I could have added Brock, May , Ritchie or a zillion other possible characters, but instead I decided to start off with what works first for the protagonist. Even too many friends can end up being abit overwhelming XD.
Then ofcourse, as the journey progresses, friends come and go, be it permanent or temporarily, its all based on what fits the situation. For instance, Misty will during a short period of time sometimes travel around with May to improve May´s water skills.
While Ash ofcourse will miss her like hell, he gets Brock in exchange, and takes the oppertunity to help Ash polish his rock tactics or maybe Pokemon diet and such.
So short answer, as many as needed to make it interesting but as low as possible so give characters time in the spotlight or end up being just there as a drag.
Hope I have helped out abit with my views