My understanding is that all RX 480 cards (also RX 470 since this is also Polaris 10 architecture) can support 6 displays at 1920x1080. But I just spoke with a rep at MSI about the MSI Radeon RX 480 4G which lists max displays as 4. Rep confirms this, but he didn't sound like he was familiar with max multi-monitor setups. The card has 3 DP ports and 1 HDMI port, so I would have guessed you could connect 6 DP monitors (using MST hub or daisy-chainable monitors) or 5 DP monitors and 1 HDMI monitor.
I have an even older Radeon R9 200 series card (DP 1.2 vs DP 1.4 of RX 480) and it can support 6 monitors.
Does AMD maintain a database showing how many displays each card, whether AMD reference or vendor, can support?