Question: I will be playing my first 5e session here soon. I got to skim the basic rules PDF, so I know that it plays a bit like 3.5 with some modernization. (*Please don't stone me if I'm oversimplifying that*.) I'd like to know what I should bring in my dicebag.

*I know that it's usually best to have one of each dice in D&D, regardless.*

That's not the point. The point is that I'd like to know if gameplay can be improved/hastened by having multiples of any of the dice.

For example, casting *fireball* in 3.5e without a big stack of d6's was a *very slow task*.

Likewise, there were many, many instances in 4e where multiple dice of the same kind were rolled. Especially so for damage rolls on Encounter/Daily powers: [2w], [3w], etc.

**Would be wise to bring extra dice of certain types?** I'm open for **theory** based on your understanding of the rules and what **may** come up as much as I'm interested in **personal experience**.

Answer: 
The dice I always seem to be short on are d4s. For healing potions and magic missiles, mostly.

Two d20s is essential, for rolling with advantage.

But seriously folks, there's no such thing as "too many dice" :-)
This really depends on the level of the characters, if starting with level 1's then the basic 7 dice set, or pair of each is more than enough to cover you for a while. 

At level 5 you get access to extra attacks and to level 3 spells. There might be a need to add additional dice, lightning bolt/fireball damage starts off at 8d6. 

Having multiple dice is always useful for speeding up game play, but you're not throwing around massive damage spells or attacks every combat round, and with the mechanics of spell damage being based on the level slot you are not throwing around tons of dice for a level 1 magic missile. 

I play with my family and while still low level they each have their own base dice set and I have a few extras as the DM and it's been more than sufficient. A pair of each dice and a couple extra D6's will hold you over without slowing down game-play. 

I have not run any games above level 10, but a few extra dice D6 and D8 again and any dice related to your primary weapons would probably help speed things up a bit.