With resistors in series the total resistance is equal to the sum of the resistances.The path taken by the charges is increased with each resistor so the more resistors you add the greater the resistance becomes. For n resistors in series:
Rtotal = R1 + R2 + R3 + ..Rn
With resistors in parallel the total resistance drops with each added resistor in parallel because there is more than one path taken by the charges. So for identical resistors the total resistance is inversely proportional to the number of parallel resistors.The total resistance is always smaller than the smallest resistor. For n resistors in parallel:1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ..1/Rn