fix flaky test FieldSortIT (#12259)

Signed-off-by: Poojita Raj <poojiraj@amazon.com>
(cherry picked from commit 7b99274)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

server/src/internalClusterTest/java/org/opensearch/search/sort/FieldSortIT.java

@@ -197,7 +197,8 @@ public void testIssue8226() throws InterruptedException {
     public void testIssue6614() throws ExecutionException, InterruptedException {
         List<IndexRequestBuilder> builders = new ArrayList<>();
         boolean strictTimeBasedIndices = randomBoolean();
-        final int numIndices = randomIntBetween(2, 25); // at most 25 days in the month
+        // consider only 15 days of the month to avoid hitting open file limit
+        final int numIndices = randomIntBetween(2, 15);
         int docs = 0;
         for (int i = 0; i < numIndices; i++) {
             final String indexId = strictTimeBasedIndices ? "idx_" + i : "idx";