12k is a good finisher, you would want something like an 8k for the initial bevel setting, and 4k for restore. So to answer your question, if you have a good razor and just need touch-ups then a 12k is going to do just fine. I'd say just hone as soon as you find the strop not doing enough, since you will need to do a bit more work than if you also had an 8k. So long as it's nicely lapped, it should serve you very well. Each stone type will have a slightly different cutting profile, you will still have to see how yours works out, but if it still shaves, 12k should be enough to really get it singing again.