As others have indicated, this is more a matter of orchestration or arranging than it is of dynamic markings. Some of it is the perception of fullness created by other aspects when more instruments are added, such as greater rhythmic complexity, coverage of more of the frequency spectrum, and added tone colors.
Faith I.
asked 04/05/19What is this dynamic called?
What is the dynamic called when the song gets louder when there are more instruments being added in when there is no specific dynamic marking.
2 Answers By Expert Tutors

Bianca B. answered 04/08/19
Fun and Creative Voice and Music Teacher
When music gets louder it is called a crescendo. When the music is played loudly suddenly then the marking is something called forte which is Italian for loud.

Bianca B.
To my knowledge there is no dynamic for this. This is just the natural way instruments sound when played together. Hope this helps!04/08/19

Franklin D.
There's no dynamic marking or expression for this situation. But it's a common phenomenon that can be described as "the more the louder."03/17/21

Yuriy L.
This is called "doubling" in classical music or "layering" in electronic music. This does make the music louder, but it is not a dynamic, which is why there is no dynamic marking for it.03/21/21

Temidayo B.
This doesn't have a specific dynamic term.02/15/22
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Faith I.
I’m sorry, I probably wasn’t specific enough. I’ll try giving an example. So, in a song, there is no dynamic markings. But the song is getting louder because more instruments are coming in. And will get softer when instruments start to stop playing. What is that called?04/08/19