Please note that the Unown from EX Hidden Legends and the HGSS set don't count. The names of the cards are just 'Unown', and as such you can only have 4. (The Johto Unown also had a special rule on this, stating on the top right of the card you could only have 4 total Unown.)
But yes, as Craig III said, all 28 Unown from the DP sets can all be in a deck legally (don't forget the ! and ?), as the letter/punctuation is part of the name.