1) One
-> This is singular (no s, ends with an o), so there is only one person participating in the activity
2) Two
-> This is not singular, it has an "an" ending, which is used for the subject pronouns ustedes, ellos, and ellas, all of which are plural
3) One
-> This is singular (no s, ends with an o), so there is only one person participating in the activity
4) One
-> This is singular (no s, ends with an a), so there is only one person participating in the activity
5) One
-> This is singular, while it ends in an s, it is talking to someone close to you
6) Two
-> This is not singular, estudian has an "an" ending, which is used for the subject pronouns ustedes, ellos, and ellas, all of which are plural
7) Two
-> Same as number 6. This is not singular, necesitan has an "an" ending, which is used for the subject pronouns ustedes, ellos, and ellas, all of which are plural
8) One
-> This is singular (no s, ends with an o), so there is only one person participating in the activity
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly. (ノ^∇^)
- Heather