I'm going to argue that one is the best choice, and that none is not ideal.
There are very few bug pokemon that do not evolve. These are Pinsir, Shuckle, Heracross, Volbeat/Illumise, and Durant. What these all have in common is that they do not have much to iterate on, especially the small ones such as Shuckle and Volbeat/Illumise. You can argue that Pinsir and Heracross can iterate on their horns as a concept, but if you look at them from a design standpoint, there is very little in their concept already to iterate on. I can't see this design being as simple as any of the aforementioned Pokemon, especially with the clear themes it has. In fact, I would even argue that two would be fine given this design is on the same caliber as the likes of many early-route bugmons, especially Vikavolt.
Why do so little bug pokemon not evolve? The answer could stem from the creation of Pokemon itself. Pokemon, as a series, has always been about the concept of evolving monsters, and what better animals can you base a monster off of than a bug, known for their ability to metamorphose? I believe that this is why bug-types in Pokemon always try to iterate in some form, especially when compared to other types of animals such as mammals and birds which generally do not undergo such a huge shift in appearance.
For those that can't see this design iterating don't seem to understand the potential a bug has when evolving. This design has a very substantial amount of design aspects to build up to, especially that of its tail and armor-like body. Look at Pokemon like Vikavolt. It has a very clear progression in evolution and tells a story with it. I am 100% sure this design can pull something like that off, and it kind of bothers me that people here don't seem to have the mental creativity to see that it has that potential.
A very opinionated take from me, but not having it evolve would do this design and bug-types in general a huge disservice just because it looks "fine as is".