The amount payable for Additional State Pension or Second State Pension (S2P) is related to your earnings and amount paid within the limits for National Insurance Contributions NICs.
Therefore, in 2014 - 2015 for example, the amount payable is based on NICs paid for earnings between £111 and £770 per week. The amount of years you have "contracted out" will be factored in as well which means that your pension may be lower.
When you make your claim for a State Retirement Pension, any S2P due will also be calculated.
Therefore, the maximum amount of S2P you can receive is £167.40/week (2014/15 rate) but this may be revised each year.
Visit NI Direct, for more information on the additional state pension.