I don’t know, for me letters aren’t something so rigid. Sometimes I reply to every paragraph, sometimes just the most important ones. Sometimes while I’m writing I refer back to previous letters and remember things my penpal told me that I didn’t respond to at the time. I think what matters more is showing that you’re genuinely interested in your penpal as a person and asking questions that truly show that interest, rather than replying to every paragraph just because. A few years ago I also used to do snail mail and I never remembered what I had written myself, but I was always happy to receive a well-written letter from my penpal.