According to the Texas survey I noted in my reply, it's about 7.5 percent of eligible voters (therefore, not counting illegal immigrants) who do not have photo ID. I don't know if that percentage applies across the entire country though. Certainly enough to tilt an election one way or the other if all of them voted compared to none of them voting. Of course the reality of how many do/don't or would/wouldn't is somewhere in between.

What forms of photo ID are there other than driver's licenses that people can get easily? I'm asking out of curiosity, because here in Canada I can't really think of any off the top of my head that I've ever personally encountered. My only photo IDs are my driver's license and an old provincial Metis association card that expired years ago. People who don't have cars and have never driven would have a hard time qualifying for a driver's license, and people who are homeless would have no fixed address to have one mailed to. But there may well be other forms of photo ID one can easily obtain in the US.