The usual mix for dofollow vs nofollow links in a PR is about 90% nofollow and 10% dofollow by default; however, it also really all depends on which news outlets decide to run the PR with dofollow links, as they have the "final say" when inputting the HTML on their pages, so there is this chance that there may be mostly nofollow links in the PR.
You can also request all nofollow (recommended).
This does not really diminish the SEO value of your PR, as PRs are mainly used to diversify anchor text and are not a very sustainable way of link building if these were all to be dofollow links. While you will not be getting link juice from the PR, Google will still recognize these natural anchors in your anchor text profile, so you have done a really nice job at diversifying your anchors.
You can learn more about how we recommend you use a press release here.
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