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The Drain current limit spec lists it. 5 or 10 volts VIN for 20 amps. That is however with A Drain Source voltage of 13 volts. I'm not sure how to calculate it for higher DS voltages, someone else here might be able to. Personally I'd like to know. Keep in mind if there's any resistance on the low side of the mosfet it's going to increase the VIN required to full switch the FET on too.
N channel according to Google. Most fets are N channel, because P channels can't carry as much current given the same physical size. You can use a push/pull transistor setup to boot strap the main supply voltage to the gate. If the Mosfet is on the lowside VCC will always fully turn it on.
You can tell it's N-channel since the gate-source voltage is specified as positive.
The "ON" Table shows the Drain-source On Resistance as .07Ω with 5V gate-source voltage and .05Ω with 10V gate-source voltage at 10A drain-source current. From the "Output Characteristics" curve it looks like you wouldn't want to run it at more that about 15A as a switch with Vgs=10V or it will come out of saturation.
The max gate voltage is 18V, which is as "on" as it can get. You need to look at the graph for "Static Drain-Source On Resistance vs Input Voltage." This show's you that it starts conducting at ~5v, and the Rds(On) goes down as the voltage goes up.
Most MOSFETS are driven between 10-15v, 10v and 12v being most common on datasheets.