The answer to this question is all dependent on you. Let me give you some examples.
If your goal is to gain muscle or strength, then keeping the workouts gym-based and simple is probably the best for you. Make small adjustments to load rep ranges and exercise modality as the weeks go by to ensure that you’re progressing. But that’s probably as far as you need to go from a variability perspective.
If your goal is to run a marathon, then the variability comes from the type of runs you are doing. One of the runs in your typical week may need to be a longer one; one may need to have some sort of interval or incline work included; while one may need to be a recovery run at a slower pace. And you could add one or two sessions in the gym that are specifically designed to complement your training and contribute towards injury prevention. This may look very different from the example above and include a wider range of exercises.
Another example is for the gym-goers who don’t really have a specific goal in mind. To these guys the amount of variation required is entirely up to you. If you take me as an example, pretty much I do the same movements each week and change only rep ranges, load and time under tension. I enjoy this way of training as you can monitor your progress much more easily. While this works well for some people, others find it unbelievably boring. So, changing exercise modality and style every other week may be more appealing if that does seem a bit boring to you.
In summary, you don’t need to be doing loads of exercises to progress, as long as you are creating progressive overload over the weeks. Vary the routine more if you find it boring or have more dynamic goals, but remember you're not losing much (if anything at all) by keeping everything consistent.
See you next time.