Bitcoin should not be kept in one wallet, because if a single wallet is hacked, all the bitcoins will be lost. But it is definitely wise to collect bitcoins in multiple wallets, you should notice and think that the more wallets the safer they carry. El Salvador is a long-term holder so they should definitely use more wallets, using 14 wallets they hold 6285 bitcoins.
In my opinion, they should use more wallets, because they are constantly collecting bitcoins so the more wallets they should use the safer they carry.
The more, the better, but the more wallets there are to guard, the more likely one of them would be cracked (due to human factor, probably).
So it's a question that would need the balance from both parties.