I've never known this for if I ever want to found a clan. But in order to be called alive as an unofficial clan, do you need 5 active members or 10 active members?

You need 5 members to war. So I'm assuming five.
Also. I don't think a unoffical clan can be considered offically dead or alive. Because there are no real consequences for having an inactive unoffical clan. I'm not 100% although.

@Val Your clan can be deleted.

Under some specific circumstances such as freeing up the name. Then yes. But that doesn't happen often.

An unofficial clan can have 1 member and it'll still be considered "alive".
A clan must have 5 members for it to be able to utilize some of the functions (warring, making ranks). But, a clan must have 10 if it's going to apply for official.
Clan Discussion & Guidelines should clear up other thoughts on the topic.