Constant solana_runtime::block_cost_limits::MAX_WRITABLE_ACCOUNT_UNITS

pub const MAX_WRITABLE_ACCOUNT_UNITS: u64 = MAX_BLOCK_REPLAY_TIME_US * COMPUTE_UNIT_TO_US_RATIO; // 0x0000_0000_00f4_2400u64

Number of compute units that a writable account in a block is allowed. The limit is to prevent too many transactions write to same account, threrefore reduce block’s paralellism.