Any hints on basic team building? Like how many defenders or leaders do I need.
You need a striker to win matches. It's recommended to build a team around a striker. If you didn't start with one, you'll have to invest GP into getting one from the scout.
Littres, +'s and scout priority is: One Striker >> One GK ≥ One Passer > The rest.
Depends on your type of striker you'll want to shift your priorities from GK to passer.
Strikers, for the most part, land into two different camps: One-shot and Utility. One-shot types use crit and high attack power to hopefully one-shot a goalkeeper. They will generally want an assist that grants extra damage and critical. Some examples are: Beatrice, Sharr, William. Utility will require more shots to kill but implore some kind of other way to kill or recover the ball, like action bar manipulation or weakening enemy defenses over time. Examples are: Ravian and Kirin.
Once you get a good striker, you'll need some way of not losing the game, so get some kind of goalkeeper to take a few hits. They're a pain in the ass to find thanks to the healing nerf a year ago invalidating a lot of goalies in PvP. Still, anything is better then nothing, so pick one of those up, even if you need to use an off color or a defender that doesn't specialize in goalie. They just need to not die in one shot.
After that it's a passer/assist to help your striker in some way, passive line buffs for example or in the case of one-shot strikers, a lot of extra crit/attack power to overpower the defenders. A good passer should be able to keep up with your striker's speed for PvP(I'm not one to talk since my passer is very slow and Beatrice is extremely quick). Pass skill/passives should support the striker's playstyle.
After that, some kind of totem, which is basically a character that supports the rest of the line with buffs/debuffs of some kind. Yuri is the best definition of that with 3 different stacking line buffs. A totem doesn't have a specific typing, it's just some kind of character to bring some kind of killer buffs to your striker/team to help them do their job. Some examples are:
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Yuri(Attacker), with extra damage, pass, HP and crit resist for all her allies in the same position.
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Nerua(Defender), with her damage resist and extra hp made any back line super tanky. She was nerfed a lot for off color teams, so she isn't as good to splash anymore but still alright if needed.
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Kevin(Leader), helps your line act faster, enemy slow, damage resist and weakening auras. Can easily allow your line to take a lot of additional actions before the enemy gets one turn.
A lot of times, if you lack a good CM/mid field, you'll just have to give up winning the ball after the kick off. Instead, beef up your backline to take some hits with line buffed of whatever you can muster. Long passes are really nice to use early game and PvE to ignore the middle line all together, as you can focus on a beefer back to take a shot and delivering it to the frontline so you can work your magic.
A few other things because I could talk your ear off all day with stuff, so I gotta end it somewhere:
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Having too many leaders leads to never being able to push past. If you lack a long passer/forward passer, you'll just never move forward.
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Having a least one attacker in mid on the sides to punch a hole in their formation is good. The CM position could be guarded by 2 people and having to 2v1 is a bad idea, as both will have a chance at stealing the ball instead of a 1v1.
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Have a striker on the side. If the other side places their extra player in the back, your striker will have to split their damage 3 ways. On the side it will always be 2 and never more. Most strikers like to be placed on the side anyway. You also have the benefit of having one enemy unable to act on the striker, as they'll be too far to block or steal from, making them waste their action.
-Don't worry about the bonuses they get from placement, that's just a nice thing to have. Line buffs/passive/synergy should take priority.
As much as I'd like to keep talking as I said above, I need to cut it off somewhere but you can always check out /ssg/, the SS sub-reddit or the Wikia, those were the most active places if you need some information.