The number of unstable thumbtacks would depend on the specific context and conditions. Thumbtacks might become unstable if they are disturbed or if the surface they are placed on is not stable. Over time, if there are external forces or vibrations acting on the thumbtacks, they could become unstable and fall over. However, without specific information about the environment or circumstances, it’s challenging to predict precisely what will happen to the number of unstable thumbtacks as time passes.