There are 30 metal flowers in the game, scattered in various locations in the map. Like the other major collectibles, you can buy the collectibles map from any of the merchants in the starting area, as soon as Aloy becomes a Seeker. This collectible map will only show the approximate location of the collectibles, not the exact spot where they sit.
Some of the metal flowers are found in open spaces in the wild while some are hidden inside bunker ruins or what the natives in the game refer to as Metal Worlds. To collect them, simply get up close and interact with them.
These flowers can be traded to a specialty merchant named Kudiv in Meridian. The flowers are divided to three sets, with 10 flowers per set. Once you complete a set, you can trade them for a good amount of shards and mods.