One certificate, one piece of paper
The mark this errand produces is fastened to a specific sheet. It does not attach to your results, your record at the institution, or the idea of the transcript in general. That sounds like a technicality until somebody abroad keeps the certified sheet they were given, and you find you have nothing certified left for the next application.
Why the count decides the total
Two figures are published, and both attach to the document rather than to the batch: £45 per sheet by standard post and £25 per sheet on the next-day line reserved for registered businesses. Four sheets in one envelope are four charges. Nothing about sending them together reduces that, so the count you choose at the start is effectively the price of the whole errand.
The same arithmetic applies to time. One round trip through the post covers however many sheets you send, while a forgotten fourth copy means starting the journey again from London and waiting through it a second time. Deciding the number early is worth more than any choice of service level.
Sheets come from the awarding body
Every copy has to be issued by the institution that granted the qualification, carrying the signature and seal that institution applies. The certifying desk reads exactly those marks. A transcript downloaded from a student portal and printed at home shows an image of them, which is a different object and gives the desk nothing to confirm.
Keep each finished sheet intact
Once a sheet comes back it is a single certified item, and the useful habit is treating it that way. Photocopying it afterwards produces an ordinary copy showing a picture of the certificate, not a second certified transcript. Whether the certificate is enough on its own at the far end depends on the country the file is going to, since some destinations run a further consular stage after this one.