Add maximum number of queries to run

Throw error if user tries to run more than that.

extensions/ql-vscode/src/extension.ts

@@ -444,7 +444,12 @@ async function activateWithInstalledDistribution(
     commands.registerCommand(
       'codeQL.runQueries',
       async (_: Uri | undefined, multi: Uri[]) => {
+        const maxQueryCount = 20;
+        try {
           const [files, dirFound] = await gatherQlFiles(multi.map(uri => uri.fsPath));
+          if (files.length > maxQueryCount) {
+            throw new Error(`You tried to run ${files.length} queries, but the maximum is ${maxQueryCount}. Try selecting fewer queries.`);
+          }
           // warn user and display selected files when a directory is selected because some ql
           // files may be hidden from the user.
           if (dirFound) {
@@ -458,6 +463,9 @@ async function activateWithInstalledDistribution(
           }
           const queryUris = files.map(path => Uri.parse(`file:${path}`, true));
           await Promise.all(queryUris.map(uri => compileAndRunQuery(false, uri)));
+        } catch (e) {
+          helpers.showAndLogErrorMessage(e.message);
+        }
       }
     )
   );