Invisalign Trays Don't Match How Long the Doctor Said It Would Take

The number of the initial batch of trays usually does not correspond to the treatment time that the doctor told you. After you have gone through that initial group of trays which tends to accomplish the bulk movement, there is usually a period of refinement, where new impressions will be taken and you will get more trays to fine-tune your tooth movements.  

I have heard before with braces that you take the orthodontis's estimate and you double it.  With Invisalign you are limited to the trays and possible refiinements.  The refinements are often difficult, time consuming, and unpredictable, and therefore, may take more time than expected.  Good luck with the treatment.

There are things called mid-course corrections, meaning before treatment ends things need to change.  They don't ALWAYS happen, but may.
If you were given aligners and the total is 14, and you change every 2 weeks, then you should be done in 7 months (2 per month).  That is assuming the teeth move as predicted by the computer.