Q: How many Owl Statues are there?
A: There are a total of 5 Owl Statues.

Q: When will all the Owl Statues be accessible?
A: I recommend to search for them after you've completed the following quests as they will unlock some paths to these owls:
1. Echoes of the Past (Bellara)
2. How to train a Griffon (Davrin)
3. An Unfamiliar Sense (Harding)

Q: What do I get for activating all the Owl Statues?
A: Aside from some loot and notes, these Owl Statues do not give anything. (the note is especially awesome!)

Please keep in mind that these are made in no particular order!

Local map location:

In-world location:

Owl's reward:

Local map location:

In-world location:

Owl's reward:
An amazing note.

Local map location:

In-world location:

Owl's reward:
Another amazing note.

Local map location:

In-world location:

Owl's reward:

Local map location:

In-world location:

Owl's reward:

Hope this helped you on your journey to find all the secrets in Arlathan Forest.
Always open to feedback! :)