i'm planning to apply visa for my wife so she could join me here in the UK. what is her chance if i'm only earning £14k, got a mortgage and a british citizen?

Providing that 14K is sufficient for you to meet your financial obligations and to provide for both you and your wife, there's no reason why she she shouldn't apply for a visa. The visa officer will have other considerations, but there is no set figure required to demonstrate the ability to financially maintain someone.

Scouse.

Quite. There is no set figure.

From DSPs Chapter 9 – The maintenance and accommodation requirements

There is no prescribed minimum figure for what represents sufficient maintenance. Appeals Tribunals and the higher courts have consistently upheld the principle that the appropriate level of maintenance without recourse to public funds varies widely from case to case

Provided you are earning enough to meet your commitments (rent/mortgage, utilities, servicing debts etc.) with some left for day to day expenses such as food, then you will be fine.