What one set holds
A finished set is three things fastened together: the new English text, a copy of the sheet it was made from, and a signed, dated certificate of accuracy. A certified translation into English in Leeds is meant to be handed over whole, because the certificate names the document it belongs to and loses its point once it is separated from it.
Who ends up keeping one
Anywhere that opens a file tends to keep what it is given, and you should not count on getting it back. That is the practical reason the count matters: each reader who files a copy has taken one set out of your hands for good. Working out who those readers are is easier before the job starts than after the first set has gone.
Counting before you order
The sum is short. Sides in one document, times the number of sets, at the per-page rate. A certified translation for Kenya covering a two-sided certificate is four sides across two sets, and the figure moves in steps of that size. Deciding it early also keeps every set identical, signed on the same day, with nothing to explain later.
Adding one later
A later run is perfectly possible and is simply a new job: the same source sheet, a fresh signature, a fresh date on the certificate. What it costs you is time, since the standard window applies again. Where a deadline is already close, that is the moment people wish they had counted one higher at the start.