The TCP “acknowledgment number” contains the next sequence number that the sender is expecting to receive (so basically senders determine the next sequence number). This is the current sequence number (for the other host) + the number of bytes in the data segment of the packet being sent to that host.
Packets with a zero-length data segment that start or continue a conversation (for example, SYN packets) get their sequence/acknowledgement number incremented by 1. This is called a “ghost byte”.
The acknowledgement number for RST packets is always 0.
The initial SYN packet that starts the three-way handshake should not have an acknowledgement number