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; // 12_000_000u64

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, therefore reduce block’s parallelism.