In Setup | Settings | Payroll | Incomes, if you scroll down you should see 12 more, a total of 20 incomes.

However it may not show up on the printed paystub - even tho the calculations are done correctly. You may want to do a test for the 8 or more that you do have and see if all gets printed.

If there are some categories that are not used for a particular employee, you should probably un-select them so that they aren't used accidentally.  

Any zero YTD balance incomes just don't print on the stub, even if they are enabled.   For instance, 'Bonus' is enabled on my employee record, but there was no Christmas in July this year.  

Sage 50 doesn't taunt me with 'Bonus......... $0.00'   all year.