Constant LUMEN_DEFAULT_THREADS 

pub const LUMEN_DEFAULT_THREADS: usize = 4;

Upper bound on the default worker count for the bevy_ecs task pool when no plugin or env var raises it. One worker per main-stage band (input / systems / layout / render). The effective default is default_thread_budget = min(available_parallelism, LUMEN_DEFAULT_THREADS), so a 24-core box does not spawn a 24-wide pool for a UI that never saturates four workers.