In theory, the port range is between 0~65535, so the max value can be 65536 depending on the customer's data or any custom parsers customer have.
You may observe "Metakey max capacity reached" messages in /var/log/messages as well. This warning message is basically telling that the maximum capacity has reached and it will not take any new values. But it doesn't mean that there are more values in this case.
For your information, you can monitor specific meta values inside index slices by using inspect in concentrator service.
Port range between 0 to 65535 is actually 65536 unique values. Therefore, you need to change valueMax of tcp.srcport to 65537 to resolve this issue. In spite of the data type(UInt16), the valueMax can be set for this key and it won't create any problems. In Netwitness version 10.5 above, this value is set to 65537 by default.