Make roll method take an int: number of dices to be rolled.
Use dynamic arrays (or vector) to store any numbers of roll.
Support of different sided dices (d4, d6, d8, d10, 1d2 ,d20, d100...) is a godd idea.
that about flexibility.
You should use random generator classes in <random> and seed them once per instance of Dice, preferrably in constructor.
srand(time(0)+rand()); does not give you more randomness than just srand(time(0));.
Also you can calculat Low and High dot in roll method and store them in private variable, making highDot and LowDot an accessor functions.