A spell is modal if it has two or more options in a bulleted list and instructions for a player to choose a number of those options, such as "Choose one" or "Choose one or more." Each of those options is a mode. Spells with spree are modal spells.

Modal spells printed prior to the Khans of Tarkir™ release didn't use bulleted lists for the modes; these cards have received errata in the Oracle card reference.

The X in Riku's ability is the number of times you chose a mode for the modal spell you cast, not the number of distinct modes you chose. For example, if you cast a modal spell that allows you to choose the same mode more than once and you choose the same mode three times, X is three and you may choose up to all modes of Riku's ability.

You can't choose the same mode of Riku's triggered ability more than once each time it triggers.

You pay all costs and follow all normal timing rules for cards played from exile with Riku's triggered ability. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.