Let's see then.
According to the table, the maximum IQ a Pokemon can have in Explorers of Sky is 999.
This is how each type of Gummy will affect your Pokemon's IQ. Since you didn't mention which type of Gummy you're referring to, I'll take it to be neutral.
Now that means that the amount of neutral Gummies needed to Max out the IQ a Pokemon has would be = 999 / 3
Also IQ is not affected by the group the Pokemon are in, they affect the skills a Pokemon learns at a particular IQ / level.
Source: Bulbapedia, and some Unitary Method.
Hope I helped!