Is there a rule of thumb regarding the size and number of a buffer pool instances?
BUFFER POOL INSTANCES
The rule of thumb I usually use is based on a special program in in the Linux environment
numactl --hardware

When I run this, I get the following output
sh-4.1# numactl --hardware
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3
node 0 size: 49151 MB
node 0 free: 241 MB
node 1 cpus: 4 5 6 7
node 1 size: 32768 MB
node 1 free: 39 MB
node 2 cpus: 8 9 10 11
node 2 size: 49152 MB
node 2 free: 49 MB
node 3 cpus: 12 13 14 15
node 3 size: 32752 MB
node 3 free: 32 MB
node distances:
node   0   1   2   3
  0:  10  16  16  16
  1:  16  10  16  16
  2:  16  16  10  16
  3:  16  16  16  10

This quickly tells me how many CPUs and cores I have on my DB Server.
In general, I set the innodb_buffer_pool_instances to the number of physical CPUs or the number of cores. In your particular, I set the innodb_buffer_pool_instances to 4 or 16.
BUFFER POOL SIZE
If you make the InnoDB Buffer Pool bigger than 50% of the installed RAM, mysqld causes the OS to start swapping ... BADLY !!!! If you need a Buffer Pool that big, then tuning innodb_buffer_pool_instances becomes even more critical.
EPILOGUE
As soon as innodb_buffer_pool_instances was first introduced, I immediately experimented with it. I had a client that had 192GB DB Server, dual hexacore with a 162GB Buffer Pool. I simply set it to 2 and everything worked out just fine. Please see my old post from Feb 12, 2011 on this : How do you tune MySQL for a heavy InnoDB workload?
GIVE IT A TRY !!!