AFAIK the limit is in the kernel itself, as compiled by Synology. The platforms enabled by the current loaders have the following compute characteristics:
DS3617xs - Xeon D-1527 - 4C/8T
DS3615xs - Core i3-4130 - 2C/4T
DS918 - J3455 - 4C/4T
There has been some confusion about cores vs. threads. I think that 16 threads is the kernel limit.
As you can see, 16 threads covers all these CPUs and we have evidence that 16 threads are supported on all three platforms.
If you have more than 8 cores, you will get better performance by disabling SMT. @levifig, you are already doing this. I don't think there is any other way to support @Yossi1114's 10C/20T processor other than to disable SMT.
If someone wants to develop a loader against a platform with more thread support, may I suggest investigating the FS3017 (E5-2620v3 x 2 = 12C/24T) FS2017 (D-1541 = 8C/16T) or RS3618xs (D-1521 = 8C/16T). It would stand to reason that the kernel thread limits might be higher for those platforms.