The Trial version of Vizard does not impose any limitations on the number of textures you can apply to an object. What kind of graphics card do you have? Are you using a fragment shader to blend the multiple textures? I know most nVidia drivers only support 4 texture units when using the fixed function pipeline to combine the textures. However, depending on the driver, a lot more units are supported when using a fragment shader to combine the textures.
Run the following script. The first number it prints out will be the maximum number of fixed function texture units supported by your driver. The second number will be the maximum number of fragment shader texture units supported.
Code:
import viz
viz.go()
print viz.getOption('gl.max_texture_units')
print viz.getOption('gl.max_texture_image_units')