In the article "One Roll, To Go", in Dragon issue 113 (September 1986), pages 74-75, Larry Church describes how to replace lots of d20 rolls with one d% roll by using the binomial distribution:

You're running a game in which the party is ambushed by 45 archers. Arrows begin flying; you resign yourself to rolling your 20-sided die over and over (and over and over and . . .). Anxious players drum their fingers as they await their turn. Is there a quicker way? Obviously there is, or you wouldn't be reading this article.

The following JavaScript calculates tables that will greatly expedite the die-rolling process. You can replace N d20 attack rolls with one d% roll. The tables are calculated from the cumulative distribution function for the binomial distribution with parameters N=the number of attackers and p=the attackers' to-hit probability (computed from the target number they need to roll to-hit on a d20).

Example: In the above scenario, the DM determines that 10 arrows are being aimed at Francis the Cleric. The DM consults the 10-Roll Binomial Table. The archers need, say, a 12 to hit, and the DM rolls a 39 on percentile dice. Look down the "12" column. Since 39% is less-than or equal to 50.44% (corresponding to 4 hits in the leftmost column) but greater than 26.60% (in the same column, next row down), the result is 4 hits out of the 10 attempts.

N.B.You can always roll your d% a couple extra times to generate 100ths of a percent, if need be.

The following JavaScript will compute an N-roll binomial table: