18 is far too many. Most terrains in UT3 as an examples used around six textures total. The trick is in how you handle your normals and specularity. The limit to the number of textures is 16 - however around 4 of these are occupied already by stuff the landscape does behind the scenes, so you only really get around 12.
At work we've run into this all the time... It's not necessarily the total number of textures in the material, that's why the material editor will not error... I think it only looks at the first layer with the highest "preview weight". Note that the texture sampler limit in the mat editor will change from 15 to 12, once…