With a few people telling me that Tumblr will only use the first five tags on a post, I emailed Tumblr support and asked them if this was true. Their reply was prompt and informative so I’ll share it below.
That is correct.Only the original post (rather than reblogs) will show up in a Tumblr-wide tag. So if you reblogged a post and tagged it, you won’t be able to find your reblog on that public tag. Also, if you’ve made multiple posts in a given public tag, only 3 will show up per page of that tag. Also keep in mind that if you’re looking for your post on a very active tag, it could already be far down on the tag page. You may need to scroll for a long time to get your post, if hundreds of other people are posting items with that tag too at any given minute of the day. If you use your mouse to hover over the top right-hand corner of any post, where the little corner flips down, you’ll see the time that the post was published appear. Note when you published your post, and then you can use those timestamps to scroll all the way back to where your post should be.Another thing to consider is how many tags you added to your post. Only the first five tags will be indexed in public Tumblr searches, and the first 20 tags on your post will be indexed on your own blog’s tag pages.