One. But does it matter? They are tasked with creating a concept, unlimited power not a concept.
A slight tangent, but my daughter and I introduced her friend, Iliany, to D&D and we started a campaign with the 3 of us. Iliany's first character was a sassy Noble-born Wood Elf Druid. We recently started a campaign with my daughter, Iliany, and another friend, Amber. When we were making characters, I told Iliany that this was the perfect opportunity to try playing something different. So she said she wanted to be a Noble because she wanted to be sassy (sounds vaguely familiar). I asked her if she had chosen a Race and she said she wanted to be a Half-elf (okay, a slight variation there). I asked what she wanted to play for a Class, and she said, "I don't know, I'm thinking . . . maybe . . . I want to play a Druid," (so much for trying something new).On a side note, this is their 3rd time playing? They might want to explore the classes and play more than 2 of them before having you be super flexible to whatever they can come up with. It seems like you are at a different point in experience than they are and to them more everything is still new.
Tiefling Rogue (bonus int and cha) using minor illusion or presdigitation to give their rapier a flame effect?It matters cause some concepts are not mechanically possible at lv1. You don't start with all class abilities at lv 1 and then those abilities improve as you level up, instead you gain new abilities as you level up.
Let take example. Daring swashbuckler duelist with classical setup of rapier and main gauche that uses his innate magic abilities and channels it into the weapon to make weapon flame up or can use that magic to boost his defensive or physical capabilities. Outside of combat he is scholar versed in various subjects who likes to spend his nights pursuing various paramours. This D'Artagnian rip off is hard to pull of mechanically as level 1 character.
Do you have a Jenga tower? If so, look up a game called Dread. Basically, you tell a story and have players pull from the tower when they want to do something with any risk at all - and if it falls their character finds disaster. The key elements are that you don't really need to have any prework or paper characters - you can just jump right in and storytell with whatever premise you want....I have players who are mostly comfortable being creative inside boundaries. So I’m looking for decision tree ideas that instead of reducing, expand, or prompts, or EASY questions that draw out their creativity to build something unique.
Ideas?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.