The maximum number of out-messages per transaction on the TON network is limited to 255 for performance and optimization reasons. This limit is designed to prevent network congestion and ensure that the network remains efficient and responsive during periods of high transaction volume. In-messages do not have this limit and can be included in a transaction in any number.
Any transaction is either special or triggered by one incoming message. In-messages can be included in block in almost any amount, until they hit block gas limits.