How many friends I can tag on my Facebook photo?
Can I tag all of my 1000 friends on a single photo?
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A 800x600px photo would display each of you friends maximal on a average of 480px. For example a area of 20x24px for each friend. I don't think that one could identify all of their 1000 'friends' to tag them. So answer is no!
According to this Facebook Help Article, 50 photos.
You can tag up to 50 people or Pages in a photo. Some tags may not be visible if there are tag requests pending. If you have reached the limit, an existing tag will need to be removed by you or the person who posted the photo before another tag can be added.