code: public static int countPaged(String keyspace,
                                   String columnFamily,
                                   ByteBuffer key,
                                   SliceQueryFilter filter,
                                   ConsistencyLevel consistencyLevel,
                                   ClientState cState,
                                   final int pageSize,
                                   long now) throws RequestValidationException, RequestExecutionException
      {
          SliceFromReadCommand command = new SliceFromReadCommand(keyspace, key, columnFamily, now, filter);
          final SliceQueryPager pager = new SliceQueryPager(command, consistencyLevel, cState, false);
  
          ColumnCounter counter = filter.columnCounter(Schema.instance.getCFMetaData(keyspace, columnFamily).comparator, now);
          while (!pager.isExhausted())
          {
              List<Row> next = pager.fetchPage(pageSize);
              if (!next.isEmpty())
                  counter.countAll(next.get(0).cf);
          }
          return counter.live();
      }
docs: Convenience method that count (live) cells/rows for a given slice of a row, but page underneath.