As an initial qualification, I'd mention that Luther Arkwright, the one published science fiction setting, breaks from a bunch of what I'm saying below, though it arrives as a similar set of conclusions about how combat styles should work nonetheless. I'm also leaving aside "Monster Styles" since they can be created off the cuff without consequence.
1) PCs will typically have between one and two combat styles right out the game, and the slow increase in skills in Mythras means that most will either stick with their original styles or pick up at most a third. I've never seen a PC with four or more combat styles, never even heard any one discuss the possibility as a realistic option for their character's development.
2) In my experience, the typical Mythras party has PCs all come from a shared cultural background, so you'll find that most of them share the same primary combat style. But, every other character in a typical party will have a career that allows them to access a second combat style (or in the case of Mythras Without Tears, will sacrifice a professional skill choice to gain access to a second combat style). Most of these PCs will want their second choice to be unique withing the party (unless one of the combat styles is particularly good). So when you're trying to judge how many combat styles you need for the party alone, use that as your baseline assumption.
3) Though they may not realise it at the start, most PCs will eventually want one of their two combat styles to have a fairly good ranged weapon (usually the short spear), at least one to let them use a shield, and at least one with the Mounted Combat trait (even if they don't need to invoke it all that often). The more they can layer these into a single style, the more desirable or necessary that style becomes.
NB: If you're a PC and you notice your enemies are using a combat style that has a trait other than Mounted Combat, try to get your enemies to jump onto their steeds (perhaps by fleeing on your own with them in hot pursuit) and then remind your referee about capping their combat styles with their Ride skill. You won't be popular, but you will be nigh-invulnerable to most stock enemies.
4) If there's a trait that rewards a bunch of PCs using the same combat style in tandem with one another (i.e. Shield Wall, or Formation Fighting) either everyone in the party will take it as their primary combat style or else it'll fail to reach the critical threshold of three PCs and be ignored / snubbed. If you're using careers, it's extremely unlikely that three PCs will get access to, and choose, the same secondary combat style through their careers, so you have to make it available at the Culture stage. In parties with multiple cultural backgrounds, don't expect people to take these combat styles.
5) The Mythras core has just under 60 weapons in it (counting shields), but most settings use a much smaller subset - I believe there's about 13 (counting shields) in Mythic Britain, and around 25 in Shores of Korantia's combat styles (with most of the variety in a small number styles that are less common). In the Dawnlands, I went for 12 - ten actual weapons, and two kinds of shields (I am considering adding another three of four, but haven't made up my mind).
A certain amount of doubling up on weapons between styles is good (since it allows a character not to have to carry a golf bag of swords), but you don't want too much overlap since that lowers people's willingness to take it as a second style without a further incentive. And that incentive might actually convince them to take the second style and ignore the first anyhow.
In practice, I find the ideal is about three weapons, especially if you're designing a lot of styles that count shields as one of those three. That lets PCs who take two combat styles use four offensive weapons, and at least one kind of shield, possibly two, without penalty. Three weapons also helps keep the style focused - with four weapons you tend to start asking yourself "What would the most common secondary sidearm for this person be?" a lot.
I also have a tendency to create a single combat style in a campaign that allows you to choose any two weapons you want. You gain in freedom of choice by losing that extra slot. This helps accommodate the folks who really, really, really want to wield a particular weapon that wouldn't otherwise be available.
6) There's a temptation that's indulged a lot to create near-identical combat styles differentiated by culture (usually with a slightly different sidearm or . Instead, I recommend picking the common types of soldier in your campaign setting, creating a combat style for each one, and then just reusing them across cultures to save time.