I don't think there is a max job limit, unless it's some sort of multiple of number of cores. I easily get way more than 12 jobs at a time. Could it be your local preference settings or app_config?

There is a limit on the number of tasks per host in SiDoc@home:

19/10/2024 19:13:25 | SiDock@home | Sending scheduler request: To fetch work.
19/10/2024 19:13:25 | SiDock@home | Requesting new tasks for CPU
19/10/2024 19:13:26 | SiDock@home | Scheduler request completed: got 0 new tasks
19/10/2024 19:13:26 | SiDock@home | No tasks sent
19/10/2024 19:13:26 | SiDock@home | This computer has reached a limit on tasks in progress
19/10/2024 19:13:26 | SiDock@home | Project requested delay of 21 seconds
19/10/2024 19:28:15 | SiDock@home | Sending scheduler request: To fetch work.
19/10/2024 19:28:15 | SiDock@home | Requesting new tasks for CPU
19/10/2024 19:28:16 | SiDock@home | Scheduler request completed: got 0 new tasks
19/10/2024 19:28:16 | SiDock@home | No tasks sent
19/10/2024 19:28:16 | SiDock@home | This computer has reached a limit on tasks in progress
19/10/2024 19:28:16 | SiDock@home | Project requested delay of 21 seconds

But you were right in your guess: it is not set as a fixed number, but as a multiplier of the number of cores.
At the moment, it is set to a maximum of 4 WUs per 1 CPU core. More precisely, 4 WUs per 1 CPU thread (virtual core).

By the way, it would be nice to increase this limit!
It was set to 4 WU back when the project had very long COVID tasks, the computation of each of which took from 1 to 3 days, depending on CPU performance.
And 4 tasks limit were more than enough because it corresponded to the stock(cache) of tasks from 4 to 12 days of non-stop(24/7) computation.

But now, since the beginning of work on the Ebola sub-project, all the tasks are short. If they are computed on a modern decent processors it takes only a little over an hour (for example on my AMD Ryzen 5600X@4Ghz with SMT=off it takes 80-90 min of CPU time per WU on average).
And a current limit of 4 tasks per CPU core is quite a hindrance now - the maximum work cache can reach only 5-7 hours of work before hitting this limit.