Optimal Number of MySQL InnoDB Buffer Pool Instances
When I go back to MySQL 5.5, I would think about this same thing.

What I learned over those years was the following: If the Buffer Pool was bigger than half the installed RAM and innodb_buffer_pool_instances was 1 (default for 5.5), the threat of swapping was always imminent.

I discussed this before : Is there a rule of thumb regarding the size and number of a buffer pool instances?. In that post, I mentioned an example of a client who had 192GB RAM on the server with 162GB Buffer Pool. When innodb_buffer_pool_instances was 1, swapping happened. When I set innodb_buffer_pool_instances to 2, things got way better.

In your case, since the Buffer Pool is exactly half, a value of 1 may be OK. I would not chance it. I would set it to 2.

Since MySQL 5.6 has a default of 8, you shouldn't have to think about it anymore.

I will say this: akuzminsky's answer has the highest principle. My answer is just shooting from hip based on past experiences (good and bad).