16 updates have been compiled into the latest Lync Server 2013 CU release.
Microsoft intends to release Cumulative Updates (CU's) once every quarter, but as we've seen with Lync Server, its becoming a monthly cadence. Each CU is truly cumulative in that it is a large hotfix that pulls together all previously released hotfixes into a single distribution. When customers need to apply the latest fixes, they only need the very latest CU.
Once again this month, Microsoft has skipped giving the CU a number, but instead a versioning label of 8308.815. If you've been keeping records, this month's CU for Lync Server 2013 is actually CU3 with CU2 released in August.
The latest CU can be downloaded from here: Lync Server 2013 Cumulative Update KB 2809243
And, the list of included updates can be found here: Updates for Lync Server 2013
Remember, updating Lync Server is a two-step process in most cases including the actual installation and then applying the backend database updates. Those steps are also defined in KB article 2809243.