"What's the average ratio of IT employees to all employees? Is there an 'ideal ratio' for certain business sizes?"
There is no 'ideal' when it comes to number of IT staff to all employees. The ratio can vary widely – 1 IT worker to 50 employees, 1 to 100, even 1 to 250 and up!
Discussions among IT professionals will often peg an average at 1 to 50 for any organization over 250 employees. Below 250 employees, try 1 to 25.
If you're at 1 to 100 (or higher), you'll have trouble handling serious problems.