I've done 3 playtroughts to see the differences between 1, 4 and 7 samurai. All runs were made as Mori clan to make them more consistant. This guide also assumes you're not gonna farm gold, lvls and resources in multiplayer.

You can create a private game in multiplayer and play there solo. Everything you get there will be saved on your single player save as well. But you have to start a new game in single player first and unlock all characters. So if you struggle a lot, you can get some gold, lvls and gear in multiplayer before progressing in the main story.

7 characters

The "intended" way to play. This is for those who likes strategy and management of your characters. You need to pay attention to all of your samurai, their stats, spells and gear. You also need to menage your gold because there'll not be enough to make the best gear possiblle for everyone. This can become overwhelming especially to those, who's used to play only one character. Took me a few hours to switch my brain from single to 7 but if you can get through it, it's actually pretty fun.
The biggest advantage is always having someone ready to fight so if one of your characters die or get injured, you can quickly summon a different one. Also, you don't need to wait for characters to regenerate or spend money on potions. I didn't buy a single one during this playtrough and needed to wait only a couple of times on mountains and in the last castle.

Pros:
+ the most balanced game
+ always ready to fight
+ no need to spend money on potions

Cons:
- lots of stat, spell, gear, gold and character menagement (can be a + for some)
- character levels at the end of the game will be lower (I got them all to 34 lvl so it's not so bad)

4 characters

The easiest way of playing ToD. Less characters means more exp especialy in early stages of the game and easier time to menage them. You'll most likely be able to wear better gear before it even drops and get the best craftable one before mountains. There should be enough gold to make a very solid gear for at least 3 characters. But since you only have 4 characters, you'll need to wait for them to regenerate after every bigger battle or buy health potions. So making health leech weapons early game is gonna save a lot of time.

Pros:
+ the easiest way, almost impossible to lose all characters at once
+ Higher lvls than with 7 chars (I've reached 38 so not a big difference)

Cons:
- need to either wait a lot or spend money on potions early game

1 character

This can be quite easy or incredibly difficult depending on which character you choose. Melee chars will struggle a lot due to monsters dealing knockback and will get stunlocked by multiple ranged enemies. It can be very frustrating. Wizzard will take a lot of time and effort to get powerful so if you're willing to suffer early game and get probably the most powerful char late game, that's your pick. The easiest character for solo play is Ninja. He can use all weapons (except polearms) pretty well and have some strong spells early and late game. Just getting him to lvl 6 will unlock some of the most powerful spells in the entire game that'll carry your way until the very end. Archer is a pretty good choice as well. There's enough exp to get to lvl 48.

Pros
+ enough gold to make the best possible gear
+ no need to split components between multiple characters
+ no need to buy potions

Cons:
- multiple enemies will stunlock you, making the game frustrating
- need to save very often since once you die you have to load the game
- maze in Zanshin castle is besically impossible as melee char due to knockback

Hope it'll help you decide how you want to play.

Have fun!