She gave away 50 free works of her art, not to mention her time and materials. While there are certainly more that could use that kind of gift, that is no reason to think the artist should be feel obligated to continue giving. It is the height of arrogance to expect an unlimited quantity of a kindness or gift simply because there may be more demand.
Everybody wants something for nothing. If you are too stupid to not cut your arms or wrists or whatever into ribbons, then you should not be getting anything for free, unless it is a free trip to the mental hospital. Let them clear their own herd so I have more air to breathe.
It's great that she gave away 50 free tattoos. She must have had a reason for choosing 50. For example, maybe she couldn't afford to give away more than that. She didn't have to give away any, but she chose to do a good thing. She doesn't owe anyone any more than that.
While I think this is a great, positive cause, I don't think the tattoo artist should be expected to put her craft on hold to extend this offer. This is her career, after all, and I wouldn't want to put her out of business just to make a point. I think that this project creates high visibility, and is in itself a GREAT step forward for people who suffered from self-harm. I don't think she should be expected to do more; she has to continue living and creating, without being hung up on this one project.