delta pegs...I'm sure they were recommended here...are fantastic. They kept our canopy firmly in its place last June when it was getting uppity ideas about flying off. We use them for guy ropes all the time now (even when it's so not windy that we hardly need pegs at all).
Depends on what pegs come with your tent. If they are the spindly little metal ones imo they are only really any good for pegging out your groundsheet with. I would never trust them in UK weather on the guy lines.
We have a box of rock pegs which are obv essential if you are pitching on rocky ground or hard standing (site this weekend appeared to have hard standing under the grass which was odd).
Delta pegs (or delta ground anchors to give them their proper name) are great for windy conditions as they won't come out.
Having a few good long metal or wooden pegs are good for pitching on sandy sites (or for the beach).
Always take more than you need. You never know when you might need to double peg some guys or lend some to a neighbour in need.
I have a huge list of what we need to take with us - it seems extensive but have been nosying at old threads on here about what to take camping with us (2 adults/4 children) -so sort of know what I should pack - there's a lot of mention of spare tent pegs - having had a quick look on camping shops there are so many different types - which ones and how many do we need? Any help from seasoned campers appreciated!!