Depending on the operating system the number of file descriptors per process is fixed in the kernel at compile time or configured with a parameter.

In terms of what makes any process different to UNIX it would be limited really to the following...

1. does it have a controlling terminal attached

2. is it's parent dead

3. is it dead, then it's a zombie and has no memory, no file descriptors and just a minimal entry in the process list.

There are other process wide details such as priority, effective user etc, but not much that makes a process different, even case 2 merely means replace it's parent pid with '1'.

Number of file descriptors is unlikely to change.