Apex: Get number of contacts for a list of accounts

Map<Id, Account> accountsById = new Map<Id, Account>([SELECT Id, Name FROM Account LIMIT 10]);
Set<Id> accountIds = accountsById.keySet();

Map<Id, Integer> numberOfContacts = new Map<Id, Integer>();
for (Account acc : [SELECT Id, Name, (SELECT Id FROM Contacts) FROM Account WHERE Id IN :accountIds]) {
    numberOfContacts.put(acc.Id, acc.Contacts.size());
}
System.debug(numberOfContacts);