There are
charts on the internet that will tell you the amperage a particular wire size can carry at a certain voltage.
You can think of resistance (measured in ohms) and amperage (the amount of electricity) as opposites on a teeter-totter. If resistance goes down amperage goes up and vise versa.
Lets pretend that each of your 5 circuits is carrying 10 amps of current. That means you need a wire (and fuse/terminals) large enough to handle 50 amps. When wired in parallel, the resistance goes down so the current goes up.
to add, in DC electricity all the electricity travels through the + and the - so the wire gauge should be consistent on both sides of the circuit.