Multiple files are not stored in a single block. BTW, a single file can be stored in multiple blocks. The mapping between the file and the block-ids is persisted in the NameNode.
According to the Hadoop : The Definitive Guide
Unlike a filesystem for a single disk, a file in HDFS that is smaller than a single block does not occupy a full block’s worth of underlying storage.
HDFS is designed to handle large files. If there are too many small files then the NameNode might get loaded since it stores the name space for HDFS. Check this article on how to alleviate the problem with too many small files.