SMP has been supported since OpenBSD 3.6, released in 2004.
Presumably the old limit was 4 cores or 16 cores.
The easiest way to get SMP support is to install the
bsd.mp kernel. That should happen by default if your system has multiple cores.
OpenBSD SMP project
As of June, 2004, SMP (Symmetric MultiProcessor) support has been merged into the main OpenBSD development branch. At this time, the i386, amd64, mvme88k, sparc64, macppc and hppa platforms support SMP.
Starting with OpenBSD 3.6, OpenBSD/i386 supports most SMP (Symmetrical MultiProcessor) systems. To support SMP operation, a separate SMP kernel (bsd.mp) is included with the installation file sets.
What OpenBSD systems support SMP?
A separate SMP kernel, "bsd.mp", is provided with the install file sets. If multiple processors are detected at install time, is automatically installed as the default boot kernel, /bsd, and the single processor kernel is renamed /bsd.sp.