The limit of 384 maxusers for auto configuration was only imposed on 32-bit systems. Currently its grow slowly above the 384 mark.
This optional directive is used to configure the size of some
kernel data structures. The parameter number can be 0 (the
default) or an integer greater than or equal to 2. A value of 0
indicates that the kernel should configure its data structures
according to the size of available physical memory. If auto
configuration is requested, the kernel will set this tunable to a
value between 32 and 384.
As explained in tuning(7), this tunable can also be set at boot
time using loader(8).