You can plug in more than 2 screens if the third and above screen have a native Displayport connection.
If you want to run 3 monitors, at least one of them must be connected via Displayport. The other two can be connect via HDMI, DVI or VGA (either via adapter or native connection)
Yes. VGA, HDMI, DVI and Displayport all support 1920x1080p at 60Hz.
Though there may be some monitors that do not support it on certain connections.
1 Monitor per adapter and it doesn't matter if it's a Displayport to DVI, HDMI or VGA, as long as it's an active adapter.
You can use all your Displayport connections and up to 2 other types (DVI/VGA/HDMI) of connection at the same time.
The total number of monitors you can use with an eyefinity compatible graphic card is usually 3 or 4, though there exists "Special Edition" cards that supports more. To the best of my knowlegde most, if not all, HD 7870 cards available with 2 mini-DP connectors supports a total of 4 monitors.
On such a card you can run 2 monitors over both the Displayport connectors, 1 monitor over the HDMI and a fourth over the DVI/DSUB.