How many Irisalis Petals are on the map?

One of the eight sits behind a barrier that stays up until the level 50 main story quest. Until you reach that point in the story, seven are reachable and the last one waits behind the force field.

You do not have to hunt for them blind. The event menu marks the points on the open world map, and the gathering petal task shows a photo of each location, so you can match the spot when you get there. Because the eight are permanent, there is no deadline on picking them up.

Why eight petals will not get you Irisalis

Collecting enough Irisalis Petals reaches milestones, and the legendary Irisalis encounter is one of them. Eight map pickups are a small part of that total. Petals also come from defeating each Alpha Aniimo for the first time, which costs no energy, from a region's silver collection badge at 70% progress, from Elite Pathfinder Challenges, from Windchasers Departure journey quests, and from weekly rotations and the shop. Those sources continue through the season, including after the event is completed.

If you are short, open the Legendary Journey event screen. Its 'How to obtain' section lists the ways you still have left. Expect the grind to be slow if you lean on permanent sources alone.