How Many Fans Can I Plug Into The ASUS B85MG R2.0 Motherboard?

According to your mobo's manual, although its two CHA_FAN headers have four pins, they actually operate only as 3-pin headers using Voltage Control Mode. That means they can power and control either 3- or 4-pin fans (because the new 4-pin fan design can work with a 3-pin header for backwards compatibility). This also means that you can use Splitters to connect more than one fan to each such header, but you can NOT use a Fan Hub.

How to tell the difference? A SPLITTER has one arm with a female 3- or 4-pin connector to plug into a male mobo fan header, and then two or three output arms all with male connectors (with pins) to connect your fans. If you look at those closely, only one of these has all its pins; the others have Pin #3 missing. That is because a mobo fan header can accept only the speed signal coming back from ONE fan, so a good splitter simply does not connect more than that. A splitter connects all its fans in parallel to the power from the header, and the total load on the header is the simple sum of the amperages consumed by each fan connected to that header. Mobo headers generally are limited to no more than 1.0 amps (per header), and today's common case fans typically consume max 0.10 to 0.25 amps each, so it is quite OK to connect up to four fans (maybe more) to each header using SPLITTERS. If you need four, get three splitters of two outputs each, and plug two of them into the outputs of the third one to make a "stack" producing four outputs from one input. These days, finding 3-pin splitters is hard, but just use 4-pin splitters - both fan types can plug into them.

A HUB may be groups of cables ending in connectors and looking a lot like a splitter. OR it may be a box with a few cables and several male outout ports. Either way, they all have one input female connector to go to the mobo header, and several output ports or arms with males connectors, much like s Splitter. What distinguishes a HUB is that it ALSO has one extra arm that must plug into a power output connector from the PSU. A Hub gets all power for its fans from the PSU, and NOT from the mobo header, so it avoids the 1 amp limit. The HUB also gets the special PWM signal from a proper 4-pin header and distributes it to all it fans so the fans themselves use it to control their speeds. BUT this means that a HUB can ONLY work if it can get a PWM signal from its mobo header on Pin #4, and that means your header MUST be operating in true 4-pin mode using PWM Mode for control. Your mobo, OP, does not do that. So you can NOT use a HUB, you can only use SPLITTERS.