I applied to 45 schools and completed 42 secondaries.
I made google documents for everything and got the prompts from school specific threads so I could pre-write them. The best advice I can give is to categorize every essay into a 'themes' which for me included: Personal growth, research interests, career in 10 years, health disparities, etc etc etc. When you get your secondaries, break up the essays into those themes and see how many of your secondary essays fit that bill. As you write, re-write, edit, and re-edit all the essays for all the different secondaries, you'll definitely get better results so go back to the ones you completed at the start early on and make sure to give them a once over before submitting.
It took a really long time, but this cycle was it for me and I don't regret it.