What happens with the voltage and amperage when we have multiple batteries in either series or parallel?
With the batteries in series, the total voltage is the sum of the individual battery voltages (a 9 volt battery is made up of six 1.5 volt cells in series).  The current capacity is the same as for one battery, as all of the current has to flow through all the batteries

With the batteries in parallel, the voltage is the same as for a single battery, but the current capacity will be four times that of a single battery as each battery is supplying part of the total current.