I have a planeswalker- Kiora on the battlefield and I played Bioessence Hydra and that creature gained 7 counters , does the counters fall away when the planeswalker die or not
No. Bioessence Hydra Has 2 abilities that grant counters. The first is a replacement effect that gives it counters as it enters the battlefield. It doesn’t do anything else after that. The second is a triggered ability that gives it counters when planeswalkers gain loyalty. It never causes you to lose counters though.
Counters always stay on a permanent unless a specific rule or ability says to remove them. Even though the number of counters here is initially based on the planeswalker’s loyalty; there is no rules connection between the planeswalker and the Hydra.