His mentor, parents and sisters probably gave him some lives. Vicky confirmed in a Warriors chat that Dark Forest cats cannot grant cats nine lives.
So, if I had to guess, I would say those 9 cats were:
- Pinestar - Leopardfoot - Sunstar - Nightkit - Marshkit - Adderfang - Swiftbreeze - Gorseclaw - Raggedstar or Nightstar
It is unknown.