In England, at all levels below National League Premier, referees are expected to follow a set cautioning procedure. That involves isolating the player, asking or confirming their name, telling them they are being cautioned, showing the card, and warning about their future conduct.
So how does that relate to match cards? It means that you need to be able to record the cautioned player's name, taking just a number isn't viewed as acceptable. If you have team sheets you could write down all of the names before the game, but you are relying on getting the team sheets in sufficient time to do this. In reality you often won't have time at grass roots, and might be lucky to get them 5 minutes before kick off. Another option is just taking the first name, then matching this to the number of the team sheet after. The safest way is to take the full name at the time of the caution.
The other thing to bear in mind is team sheets are much more likely to be accurate the higher the level of football you go. At levels where a club secretary fills them in there is a high likelihood they are accurate. When they are being filled in by a player on the touchline 5 minutes before kick off that probability of them being accurate diminishes significantly. And that is why taking or confirming the name is important: if you report the caution as Dave Jones and the actual offender is Roberto Zanetti you aren't going to be the most popular person at your CFA, as it will be abundantly clear that you didn't follow the correct procedure.
