Is it true PRK can only be performed once? If I get it now can I get it in another 20 years for presbyopia?

 There is not a limit to how many times you can have an enhancement. However, we are limited by the amount of corneal tissue. You can only take away a certain amount before you interfere with the structural strength of the cornea. In addition, We don't usually do more than one enhancement as there should be few reasons to ever have to do a third enhancement. PRK/Lasik do not currently treat presbyopia.

Best way to treat presbyopia is with Refractive Lens Exchange. Going back once more to treat astigmatism with PRK will increase your chances of having a weak cornea, which you already seem to have with two Lasik enhancements. 

Typically in my office, I feel comfortable picking up LASIK flaps for the first year or two.  For someone who needs an enhancement to their LASIK result past this time, is from another surgeon, or where there is concerned about adequate corneal thickness I suggest PRK as an enhancement.  The nice thing about PRK is that it is safer than LASIK, but it does take longer to heal.  As long as the cornea is healthy and there is enough corneal thickness remaining, multiple PRK enhancements can be safely performed.  In this way, I specifically disagree with the information that you received.  PRK can be performed for those who want to induce nearsightedness for the treatment of presbyopia as you get older.  Typically, this sets up a monovision situation where one eye is set for near and one for far.  Some tolerate monovision and some do not.  Fortunately, it is likely that by the time that you are in your 50’s, there will be better solutions for the treatment of your presbyopia.