This is the number of threads performing SSL decryption - they are separate from the regular analysis threads. 4 is a fine value, provided that the machine has enough CPUs, increasing it further won't help though.
In general, the 5 thread limit relates to packet analysis threads. The whole AMD uses a lot more threads for other tasks, like the mentioned SSL decryption, reading packets from NICs and dispatching them, sequenced transaction assembly, etc. That's why a larger amount of CPUs is required.
so we don't need to specify threads to decode each type of traffic - http/https and SOAP/XML? if we assign one AMD only process https traffic then the load will be balanced through 5 threads, right? if one AMD has mix http (20%) and https (80%) how to make sure it assigns enough threads to handle https traffic?
Correct. Decodes are not bound to particular CPUs and the AMD will equally balance traffic between all 5 threads automatically - in your case (20% http, 80% https), 1 thread would process http traffic and 4 of them - https.