If you are a pay in full card user, the math runs pretty simple.
Say you earn $48K, or $4K/mo. You try to run most expenses through the card to get whatever rewards, so the most 'regular' spending can be will be $2K or so (mortgage/rent usually can't be charged).
If we want the $2K to average a 10% usage, you ideally want open credit lines to total $20K. You can have as much as $4K on the bill and still be at 20% utilization.
The above creates a 'rule of thumb' goal of about half of one's salary as the goal for available credit. With a caution to readers, this is not advice to carry that amount in outstanding debt. If the credit agencies didn't look at utilization, 10% of one's salary would be enough in most cases.
Rules of thumb are meant as a starting point. Some people avoid cards for whatever reason. One card with a $2000 line might be enough to rent a car and hotel, and be used for little else. Others play the reward game, and this advice is geared toward that kind of heavy user.