How many solar panels do you have? What is the voltage an wattage? How far from the MPPT controller to the solar array? How amny MPPT controllers or inputs do you have?
these questions will help answer how how many wires and what size wire you will need. All solar controllers (MPPT) have a input wattage and voltage limit.
Example, I have a 800 watt array. This is 4x 12 volt 200 watt panels in series to make 48 volt & 800 watts. That is 17 amps so my wire size is a min of 6 AWG for 60 feet @ 2% voltage drop. I can go up to 10 AWG @ 5% voltage drop but I want to increase my solar array to 1600 watts. So I need to increase the voltage to decrease the amperage. If I run 8x panels that are 12 Volts 200 watts each and put all 8 in series thats 96 volts @ 1600 watts which is 17 amps. The other option is to combine two sets of 4x panels in series, which would be 48 volts @ 1600 watts and teh amps would be 33. I can still use the 6 AWG wire for 60' @ 33 amps but the voltage drop will be 4%.
I hope this example explains the reason for the upfront design before buying all the equipment and wire. See the below link for a wire size calculator.
Renogy's Solar Power Calculator Tool can quickly help to estimate your solar power requirements, calculate the size and cost of an off-grid solar system needed.
www.renogy.com