What is the maximum I can contribute to a retirement annuity?

You can invest up to 27.5% of your taxable income in a 10X retirement annuity, with a maximum tax deduction limit of R350 000 per annum. However, you can invest funds which exceed the R350 000 annual limit (for example in a 10X RA) and the excess will carryover into the following year's tax returns.  

These rules are in respect to all of your retirement fund contributions. So, if you contribute 6% of your salary to your work place fund, you can then still contribute a further 21.5% to your retirement annuity and claim the full tax deduction on both.