Evolution items can be one of the most difficult items to get. Mostly because there are five different types, and you use a total of 8 evolution items all together.
Meaning you will need a ton of luck to get all the different types of evolution items along with the right ones for the Pokémon that use them.
When evolution items were first introduced, they were rewarded to played on their 7th day spin streak, though, it only happened the first time. A theory came about that you were guaranteed an evolution item for your first 7th-day streak, which was quickly proven.
After that, trainers had tried to do a ton of research of the drop chance of these items, and other possible Easter eggs to get them faster with no luck. Receiving a lot of complaints, Niantic decided to give trainers evolution items on every 7th-day streak spin. With all the research that had been done, trainers were lucky enough to get more than one evolution item during some spins.
With the update allowing us to get evolution items once a week for spinning each day. Trainers now have an even higher chance of collect more than one evolution item per spin. Though, it still requires a lot of luck. It just so happens that because you are guaranteed items on your spin streak, yours odds are that much higher.
Evolution items without the guarantee have a total of a .14% drop rate. That’s really low! That means you will on average spin 700 times before seeing an evolution item (if you to the chance out and put it with a guaranteed every .14%). Add the 100% drop chance for the 7th-day streak spin, along with a bit of random luck. You have a chance of getting more than one evolution item.