The thicker the metal, the stronger and the more rigid it is. What thickness do you need for your application? Unfortunately, determining the appropriate material thickness for a given loading condition is an engineering question that we cannot answer. Every material, no matter how thick, will have some deflection when loaded, whether that deflection amount is imperceptible or not. The amount of deflection is dependent on the material thickness, material’s physical properties and the largest unsupported span.
There may be some online calculator tools which attempt to predict this amount of deflection.