It helps if they have the lego buttons or the V ridge to keep them from sliding.  You need flats on 2-ft of the corner blocks so the rows intermesh.

Silage guys are fearless and stack them way up high.  If you do math you can stack them 3 or 4 high in general, safety-wise.  

Weight is the enemy - any loads being transferred from a big hill of dirt farther up, or the hill getting saturated with water.  Overdig some and backfill with ‘drain fill’ which is pea gravel or clean rock.  If you wrap the back with geotextile it will let water through but not any dirt wash through if things go wrong.  Better to have soggy stuff then blocks sliding out. you’ll likely want a drain pipe like a slotted sewer pipe collect all that water away from the hillside and send it somewhere.