I'm getting sixteen divisions from The General Laws of the Eighth Legislature of the State of Texas, Chapter 89 - "An Act to provide for the Organisation of the Militia of the State of Texas" - Section 3 - "The militia of the state shall be divided into sixteen divisions....."
Approved February 14 1860.

I have also found a reference in The Handbook of Texas, Volume 1 p.523 to Francis T. Duffau being made ADC to the commander of the 14th Division in November 1860.

I had assumed the 16 division organisation was in place from February 1860 until the 33 brigade organisation was adopted in December 1861.

Phil