I made more than 10 robots. all of them had not less than 4 materials, and some had 12 materials. I can't go back and fix this. How this will effect on the game performance later ?
It's most likely that for every material the pipeline needs to change state and validate a lot of stuff. Which means you'd end up with a shaderpass for each material.
Depending on the scope of your game and the target you could get away with it. But if you start noticing performance issues it's best you reduce the number of materials. The fewer the better anyway.
If your materials just hold different textures : put all of those textures into one texture ( you'd have to move uv mapping though ). You could potentially reduce the materials to one this way.
If your materials take on different shaders, see if you can get the same effect with fewer shaders that takes several sampler uniforms to which you can pass different textures.