这可能是因为在SDK中默认情况下只返回最新的50个拉取请求。要获取所有的拉取请求，可以使用以下代码：

import * as VSS_Service from "VSS/Service";
import * as Core from "Mentioned/Utility/Core";
import * as Contracts from "Mentioned/Contracts";

const pullRequestClient = VSS_Service.getClient(Contracts.GitHttpClient);
const project: string = "YourProjectName";
const maxPullRequests: number = 100;

async function getPullRequests() {
   try {
      const pullRequests = await pullRequestClient.getPullRequestsByProject(project, undefined, undefined, undefined, undefined, maxPullRequests);
      console.log("All pull requests for project: ", pullRequests);
      return pullRequests;
   } catch (e) {
      console.log("Error fetching pull requests: ", e);
   }
}

这里使用了maxPullRequests变量来指定要返回的最大数量，你可以将其设置为你需要的任何数量。使用上述代码可以获取所有的拉取请求。