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It is completely up to you. As you can see from the other answers there are advantages to both methods. To my mind you could simply use larger paragraph breaks occasionally, rather than headed chap...
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Source: https://writers.stackexchange.com/a/33324 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
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It is completely up to you. As you can see from the other answers there are advantages to both methods. To my mind you could simply use larger paragraph breaks occasionally, rather than headed chapters with names and/or numbers, if you want to keep the flow. Avoid changing the type of novel you want to write based on the ideas above though. I'm pretty sure that if you wanted to write an epistolary novel you would already have chosen to do so.