GND on two layers

No. You would need to know the distribution of the current for DC. For

AC currents in the RF range you need to know the shape of the pulse and

solve the wave equation to know where the current will flow and what

skin depth it will flow in. Nobody does that.

For "normal circuits", it's seldom an issue. How much current are we

talking about here anyway? DC, audio or RF current?

To get a comparison for DC currents you can look at trace temp rise vs

size charts for ordinary traces. You will see that unless you use super

thin "digital" traces, it's usually okay. I always use the biggest

traces I can fit anyway. There's a ULP that calculates temperature rise

on traces for DC currents; you'll see that your plane is likely to be

much bigger than necessary. You don't need to know how many vias to add,

because you should just add as many as you can to get the best

performance. You can estimate the temperature rise of a via by thinking

of it as  a wire, figure out the equivalent wire guage, consult a

current vs temp rise site or chart to get the temp rise. You should

specify your vias to be solder filled for best DC and thermal

conductance. If you will never solder to them, you don't need thermals

on vias (EAGLE default anyway). Actually, it's possible to tack solder a

wire even on a via with no thermal, so it's rare to need a thermal on a

via. All this applies only to DC currents, RF/pulse currents are a whole

nother story...

No. You would need to know the distribution of the current for DC. For

AC currents in the RF range you need to know the shape of the pulse and

solve the wave equation to know where the current will flow and what

skin depth it will flow in. Nobody does that.

For "normal circuits", it's seldom an issue. How much current are we

talking about here anyway? DC, audio or RF current?

To get a comparison for DC currents you can look at trace temp rise vs

size charts for ordinary traces. You will see that unless you use super

thin "digital" traces, it's usually okay. I always use the biggest

traces I can fit anyway. There's a ULP that calculates temperature rise

on traces for DC currents; you'll see that your plane is likely to be

much bigger than necessary. You don't need to know how many vias to add,

because you should just add as many as you can to get the best

performance. You can estimate the temperature rise of a via by thinking

of it as  a wire, figure out the equivalent wire guage, consult a

current vs temp rise site or chart to get the temp rise. You should

specify your vias to be solder filled for best DC and thermal

conductance. If you will never solder to them, you don't need thermals

on vias (EAGLE default anyway). Actually, it's possible to tack solder a

wire even on a via with no thermal, so it's rare to need a thermal on a

via. All this applies only to DC currents, RF/pulse currents are a whole

nother story...