The number of cards you discard is equal to Horrid Shadowspinner's power at the time you chose to draw cards, even if another effect changed the number of cards you drew or in the unusual case where Horrid Shadowspinner's power has changed since you chose to draw cards
However, I put my reservations aside when I saw the cards in the Marvel's Universe bonus sheet, which will be included in both Play Boosters and Collector Boosters
As youd expect for a game thats over 30 years old at this point, Magic: The Gathering has seen some seriously broken mechanics in its time
(2/18/2022) If a channel ability requires a target, you may not activate it without a target just to discard the card.
Fun Ranking: Fun in the 99 Edgar, King of FigaroEdgar, King of Figaro Powerful card draw effect if you wanted to build a mono-blue artifact Blink deck
For example, if you have six cards in hand as Doctor Octopus's last ability resolves, and you also control Thought Reflection (an enchantment that says "If you would draw a card, draw two cards instead"), you'll end up drawing four cards