1. How many nouns did I write?
2. How many nouns have I written?
Which one is correct or more natural?
Hi, Ruifeng: Both (1) and (2) are correct and natural.
ruifeng posted:Is there a difference between American usage and British usage?
BrE speakers tend to use the present perfect more often than AmE speakers do.
ruifeng posted:And is the word "just" necessary when in a situation like this?
No. "Just" is optional in this context. That the writing (has) just happened will be obvious to the students, who (have) just witnessed the event spoken of.