Yes, you can freeze avocados. But you need to make a puree. Do not freeze a whole avocado or slices.
Puree the flesh only adding about 1 TBS for every 2 avocados. Select fruit that is not dark - but ripe and gently yielding to the touch. As with anything that is frozen you want to leave some space at the top of the container for expansion, commonly referred to as "headspace". 1/2" should be sufficient headspace. Use within 4 to 5 months maximum.
"Count yourself...you ain't so many" - quote from Buck's Daddy