As a general rule of thumb, I use one support dowel per inch of cake (so 12 dowels for a 12" cake, etc), and make sure that they’re inserted straight down into the cake ~ if they’re cut unevenly or placed in at an angle, that will cause problems! Then insert a long dowel through the middle of all the tiers. Are you using a cake board underneath each tier?
A quality cake recipe also helps, you want something that has substance and will hold up under the weight of the frosting and fondant. I also suggest using a butter-based frosting recipe (like swiss meringue buttercream) or ganache under your fondant, something that will harden up when refrigerated. I’ve had problems with sagging cakes when using a crisco-based recipe like american buttercream. I always deliver my cakes straight out of the refrigerator so they’re hardened up.
And definitely use a thicker cake board at the base of the cake! I’ve had my fondant crack before when using a flimsy cardboard cake board as the base. I love the masonite cake boards which I buy from Amazon. They’re $5-10 apiece depending on size, but re-useable if you can get the customer to return them to you :)