Its a PointerBuffer, and yet the idiomatic way to use vulkan's VkCommandBuffer is with a VkCommandBuffer object. How do I derive a PointerBuffer from a collection of VkCommandBuffer objects?
Have a look at the demo suite for some examples
https://github.com/LWJGL/lwjgl3-demos/blob/master/src/org/lwjgl/demo/vulkan/ColoredRotatingQuadDemo.java
First of all, vkCmdExecuteCommands is used to record secondary command buffers to execute as part of a primary command buffer. This is fairly advanced functionality, but if it's indeed what you want to do:
try (MemoryStack stack = stackPush()) {
PointerBuffer secondaryCmdBuffers = stack.mallocPointer(count);
// Fill secondaryCmdBuffers with command buffer handles.
// You can simply use secondaryCmdBuffers.put(i, cmd_buffer_instance);
vkCmdExecuteCommands(primaryCmdBuffer, secondaryCmdBuffers);
}