How many asm-instructions per C-instruction?

I'm not sure what you mean by "C-instruction", maybe statement or line? Of course this will vary greatly due to a number of factors but after looking at a few sample programs of my own, many of them are close to the 2-1 mark (2 assembly instructions per LOC), I don't know what this means or how it might be useful.

You can figure this out yourself for any particular program and implementation combination by asking the compiler to generate only the assembly (gcc -S for example) or by using a disassembler on an already compiled executable (but you would need the source code to compare it to anyway).

Edit

Just to expand on this based on your clarification of what you are trying to accomplish (understanding how many lines of code a modern processor can execute in a second):

While a modern processor may run at 3 billion cycles per second that doesn't mean that it can execute 3 billion instructions per second. Here are some things to consider:

• Many instructions take multiple cycles to execute (division or floating point operations can take dozens of cycles to execute).
• Most programs spend the vast majority of their time waiting for things like memory accesses, disk accesses, etc.
• Many other factors including OS overhead (scheduling, system calls, etc.) are also limiting factors.

But in general yes, processors are incredibly fast and can accomplish amazing things in a short period of time.

There is no answer possible. statements like int a; might require zero asm lines. while statements like a = call_is_inlined(); might require 20+ asm lines.

You can see yourself by compiling a c program, and then starting objdump -Sd ./a.out . It will display asm and C code intermixed, so you can see how many asm lines are generated for one C line. Example:

test.c

int get_int(int c);
int main(void) {
    int a = 1, b = 2;
    return getCode(a) + b;
}

$ gcc -c -g test.c

$ objdump -Sd ./test.o

00000000 <main>:
int get_int(int c);
int main(void) { /* here, the prologue creates the frame for main */
   0:   8d 4c 24 04             lea    0x4(%esp),%ecx
   4:   83 e4 f0                and    $0xfffffff0,%esp
   7:   ff 71 fc                pushl  -0x4(%ecx)
   a:   55                      push   %ebp
   b:   89 e5                   mov    %esp,%ebp
   d:   51                      push   %ecx
   e:   83 ec 14                sub    $0x14,%esp
    int a = 1, b = 2; /* setting up space for locals */
  11:   c7 45 f4 01 00 00 00    movl   $0x1,-0xc(%ebp)
  18:   c7 45 f8 02 00 00 00    movl   $0x2,-0x8(%ebp)
    return getCode(a) + b;
  1f:   8b 45 f4                mov    -0xc(%ebp),%eax
  22:   89 04 24                mov    %eax,(%esp)
  25:   e8 fc ff ff ff          call   26 <main+0x26>
  2a:   03 45 f8                add    -0x8(%ebp),%eax
} /* the epilogue runs, returning to the previous frame */
  2d:   83 c4 14                add    $0x14,%esp
  30:   59                      pop    %ecx
  31:   5d                      pop    %ebp
  32:   8d 61 fc                lea    -0x4(%ecx),%esp
  35:   c3                      ret