Using a Splitter to connect more than one fan to a single header is limited by the header's max current available, which in your case is 1.0 A total from EACH header. So you need to know the max current spec of each fan you propose to use this way. Many common case vent fans use 0.2 A or less, so even 4 can be done, BUT it depends entirely on what the fan specs say. And be aware that LED fans - the ones with a single colour of lights in the frame, not the many-changing-colours type - use MORE current than a plain fan. IF your fans are of the 3-pin type, their speeds can ONLY be controlled by a mobo header that uses the older DC Mode (a choice you do have available in your fan header configurations), and that also REQUIRES that you use a SPLITTER for grouping them, and not a HUB.
A HUB is a different device and can avoid the current limit of a header. But it can be used to control fan speeds ONLY if the fans all are 4-pin PWM type, and ONLY if the mobo header is configured to use the newer PWM Mode of control.
Your thread asks about using the mobo AIO_PUMP header for case vent fans. The answer is MAYBE, because some of the factors are not detailed in the mobo manual.
1. Most fan headers do exercise speed control of their fans, and your mobo's headers can be configured to do this using either the new PWM Mode (for 4-pin fans) or the older DC Mode (for 3-pin fans). My question is whether or not your mobo does this also for its AIO_PUMP header. You see, most AIO systems are designed to run the pump at full speed all the time. So IF your mobo actually restricts that particular header to providing NO speed control, then it can't work for you. BUT if the header acts "normally", then you're going to be OK. How can you tell? A simple test can do that once you have the system assembled. The normal start-up sequence for fans is that they always start up at full speed and then, after a few seconds, the mobo automatic speed control system slows the fans down to whatever is needed for the cool system. So, you can open your case, ensure that one 4-pin case fan is plugged into that AIO_PUMP header, and watch it carefully right as you start the system. If it starts up fast then slows down, your system IS exerting automatic speed control and you can use it as a "normal" fan header. Now shut down and re-connect your fans as you need.
2. Since the AIO_FAN header is intended for use as part of a CPU chip cooling system, it is very likely that any control it does do is based on a temperature sensor inside the CPU chip. Now, case fan speed ideally should be based on a different temp sensor on the mobo, but there does not appear to be any way to set the AIO_PUMP header to use that sensor. So the speeds of your fans attached to that header will be based on the CPU internal temperature. While not ideal, this is a pretty good way because there is a good correlation between CPU heat and workload, and overall system (motherboard etc.) heat generation and cooling needs.