Delta starter mega

So if I choose Eevee in the beginning and later on I get the deltas from sylvan, does she give me the mega stones for all of them or just one?

You can get all of the delta starter mega stones later in the game in different places, you don’t get all of them from sylvan no matter which starter you pick, you find them later in the game. You can also purchase them in the battle frontier from a black belt.