More drives in a RAID speeds up the read and write performance due to less downtime from seeks. Let me give you an example:

Lets say you have a RAID 0 array with 4 drives, and want to write a 2MB file. For this example, we will say the RAID stripe size is 512KB (this is way above average, but helps for the example. RAID strips are another factor that can impact performance, and should be tuned for the primary use of the RAID).

To write out 2MB, the RAID controller will break up the data into 4 512KB chunks. Chunk A goes to drive 1, B to 2, C to 3 and D to 4. These writes can happen in parallel, so the 2MB operation completes in the same time as one drive writing 512KB in a standalone setup. This write was the absolute best the RAID could do performance wise.

Now, lets change the stripe size to 256KB. That 2MB data now requires 8 chunks (A-H). Drive 1 ends up with chunk A, and E. Drive 2 has B and F. And so on. Now the write takes a little longer, because the RAID controller has to wait for chunk A to be written before it can write out E to drive 1.

Keeping the stripe size at 256KB, and expanding the RAID from 4 to 8 drives would now write out that 2MB file quicker, since there are 8 drives to accept the 8 chunks of data going out.

RAID 5 works similar to RAID 0 in the striping aspect, with the addition of redundancy data that is written in the same manner. In the first example, Drives 1 2 and 3 would write data, while drive 4 would write 256KB of redundant info to allow data reconstruction. Next write around would potentially write data to drive 1, 2 and 4, with the redundant info going to drive 3. RAID 5 is called distributed parity due to that redundant parity info being written to any drive.

This is all just a high level overview showing why more hard drives end up running faster (up to a point). It's a matter of parallelizing the workload, and reducing the main bottleneck which is seek time of each individual drive. Small data read/write operations tend to be measured in nanoseconds. Seek time to get to that data is measured in microseconds. The layers of striping, cache, and reordering drive operations are all different ways to try and minimize how much time a drive spends seeking vs reading/writing.