Most professional services carry 21%; the 10% and 4% rates are reserved for the goods and services listed in article 91 of the Spanish VAT law (Ley 37/1992), such as much of the food supply, books or medicines. When in doubt, check the law or ask an advisor before applying a reduced rate: a rate mistake is on you, not on your client.
Rather have this calculated for you?
kontora generates your tax forms box by box, tells you how much to set aside and reminds you before every deadline.