The problem with voter ID requirements is for the people that work multiple jobs and/or have long working hours. That, coupled with short opening hours for the ID offices, and long commute times to reach those offices means that some people can't get IDs even if they want to. Regardless of the number of people who are affected by all of this, voter ID measures affect mostly the very poor, the ones that depend the most on the government to get by.
Note that, all that being said, I'm not sure this is much of a problem simply by itself, because the people that have very low wages/education levels, usually don't vote anyway.
For reference, apparently more than 7 million people are working two jobs in the US.