Complex sentence: Next there is often a delay of many months (or in some subjects, over a year) before publication, particularly for the most popular journals where the number of acceptable articles outnumbers the space for printing.
Split into simpler sentences: Next there is often a delay of many months (or in some subjects, over a year) before an accepted manuscript appears. This is particularly true for the most popular journals where the number of accepted articles often outnumbers the space for printing.