As talisman says.

Take a look at your last few home games > Spectators tab. The %age bonus you receive from attendance is a guide to how well supported you are: its 5% when you can't fit any more in if its lower then you've got work to do and price is the best action.

(There's an attendance cap per level but you're nowhere near that.)

Sometimes even if prices are good they still won't turn up if your opponent is poor or your form is poor or other voodoo.

The number of fans is your total fanbase. Then there's your stadium capacity. Then there's the level Cap.