At what point is something "fairly common"? See detail.

It’s not common, but also not rare.
It’s close to common.
It’s more common than it is rare.
It is fairly common.

When it happens often, or when so many are made they are not uncommon. There is a certain 2 or 3,000 year old Roman coin that is pretty worthless because there are so many of them.

… I’m looking for a number. (I added to the OP)

There is no objective number. It’s a subjective term determined by the user.

Well, 5/10 would be fairly common.
1/10 would be less common.
9/10 would be crazy common.

“Several“is used after what number? It’s not 2 or 3 for example.

It depends on what “several” things you’re talking about. Could be anything from 5 to 5,000,000,000,000,000,000.

There is no number. It is a subjective designation.

Your question, and your details do not match.