I’d say it depends on how the formality of the trouser and your desired subtlety.
If you have a sleek, worsted trouser in a conservative, solid color, all of these options can work well. The solid look is so streamlined that one or two pleats won’t draw unwanted attention. If you like pleats, go for them.
Introducing brighter hues, patterns, and textures gets trickier. Most off-the-rack trousers are flat-fronted (i.e. no pleats) by default, so added pleats are more unusual — and more distracting.
And keep in mind that pleats are more noticeable without jackets. You want to avoid looking high-maintenance, like the dandy in high-waisted, pleated Prince of Wales pants.
Flat fronted: Is a safe bet (for casual and formal trousers) and will be most versatile for occasions.
1 Pleat: Can look streamlined and appropriately smarter than no pleats.
2 Pleats: Is more unusual and may get less wear; best suited for subdued outfits/colors.
Forward pleats: more casual
Reverse pleats: smarter (my preference for all pleats I commission)
Generally, I avoid pleats on casual trousers (though not always) and prefer single pleats for solid, worsted trousers; double pleats tend to be reserved for subtle, darker colors in more formal cloths.
To address body shape, I’d say that consideration comes secondary to the trouser’s scale of formality. However it is worth noting that double pleats can suit for heavier individuals quite well, with the extra cloth.
Body-wise, i’m on the opposite end. Being rather thin, I look exaggeratedly thin with single pleats; however, I can avoid that look with double pleats, since they add volume. Again, i’d refer to the cloth first.
In terms of advice, I would say experiment, but wisely. Ask yourself, will these pleats deter me from wearing this garment often? Hope this helps.