[suki]

The difficulty of mining slides up and down depending on the block rate.

As the profitability drops, some miners will stop, the block rate will drop and the difficulty will drop until things balance again.

[hoffsam]

But I wonder if there is some kind of minimum amount of mining required to verify X number of transactions. If there are many transactions, but few miners, then would transactions begin taking too long to become verified?

[bdr]

Yes, there is a byte limit to the size of a block. If blocks become infrequent enough due to decreased mining power, it's conceivable that a backlog of transactions would build up. At the next difficulty adjustment, though, speed would pick back up again.