You probably don't have enough ram to use more Render Multiple frames Simultaneously. 4GB per core is about the minimum for success.
AE5+ likes cores & RAM.
use 6GB as min reserved for OS.
If footage is greater than 480p (SD television) use 1.5-3GB/thread.
Not sure which processor you have, if you have a clarkdale processor i3-5xx you are kind of hosed about your only option is to update to 16gb is about the best you can do, if you have sandy/ivy-bridge i3-2xxx/3xxxx update it to i5-i7 and 16+GB of RAM, you'll have a lot more fun with it
Is that a minimum of 4GB per physical core, or 4GB per logical core?
Per any core. Where available, AE doesn't differentiate between physical and virtual cores. It just sees some core/ processor. The rest is handled by your CPU and operating system...