There is no limit to the number of Spell Counters you can have on “Magical Marionette”.
Removing Spell Counters from “Magical Marionette” to use its effect is a cost.
You can use “Magical Marionette”’s effect multiple times in the same turn, as long as you have enough Spell Counters to pay the cost.
You can destroy “Magical Marionette” with its own effect.
If you activate a Spell Card, and your opponent chains “Call of the Haunted” to Special Summon “Magical Marionette”, “Magical Marionette” does not get a Spell Counter for that Spell Card because “Magical Marionette” must be face-up on the field when the Spell Card is activated and when it resolves in order to get a Spell Counter.
Netrep Rulings:
“Magical Marionette” constantly checks the number of Spell Counters on it for the purposes of calculating ATK.