Reproduction steps:
1. Open attached project (TooManyThreads.zip)
2. Open "TerrainGeneratopmTest" scene (in Scenes folder)
3. Drag and drop "StartupScreen" scene to Hierarchy
4. Click Play

Expected result:
No crash

Actual result:
Too many threads error dialog is thrown and Unity crashes (see Error.png attached)

Reproduced with: 5.3.5f1, 5.4.1f1, 5.4.3f1, 5.5.0b11, 5.5.0f1, 5.6.0a3
Not reproducible: 5.2.5f1, 5.3.4f1
Regression introduced in 5.3.5f1

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There's an internal upper limit of maximum number of concurrent threads (2048 or so). Need to use smth like thread pooling to address the issue.