You only ever sign one original deed poll. From that single signed original, you make as many certified copies as you need — a certified copy is a photocopy that you have hand-annotated and signed to confirm it is a true copy of the original. There is no limit on how many certified copies you can produce, and almost every organisation in the UK will accept a certified copy rather than the original.
The number of certified copies you need depends on how many organisations you are notifying. For a typical name change, you should plan for at least eight: the Passport Office, the DVLA (driving licence and separately the V5C if you own a vehicle), your bank, your employer, HMRC, your GP, and two or three spares for organisations you may not have thought of. If you have multiple bank accounts, pension providers, or utility accounts, add one for each.
Making certified copies costs nothing — just photocopy your original and annotate each copy by hand. It takes a few minutes to make ten copies. The only exception to using certified copies is if a specific organisation explicitly requests the original, which is rare and should be questioned. Always keep your signed original somewhere safe at home — it is irreplaceable.