1, in the case of a small number of papers: participated in the publication of a paper as the second author;
2. In the case of a large number of papers: published X papers of a certain platform, including Y papers of the first author. (The second author's paper is in X ..)
When counting the number of personal papers, you can count the papers of the second author, but because they are not the first author and correspondent author, they cannot be regarded as the main scientific research achievements of individuals.
Generally speaking, the most important authors are the first author, the correspondent author and the final project leader. Regardless of the ranking, others will be considered to have participated in the publication of the paper, but it is not very important.