Just with respect to Paul, we can assume that he wrote many more letters than the ones attributed to him in the New Testament. On occasion, he mentions other letters that no longer survive; in 1 Cor. 5:9, for example, he mentions a letter that he had earlier written the Corinthians (sometime before First Corinthians). And he mentions another letter that some of the Corinthians had sent him (1 Cor. 7:1). Elsewhere he refers to letters that his opponents had (2 Cor. 3:1). None of these letters survives.
