An ability that triggers whenever "you cast or copy an instant or sorcery spell" will trigger once for each time that happens (C.R. 603.2, 603.6c). If you copy the same spell more than once, the ability will trigger that many times. Thus, in this case, it will trigger once by casting Grapeshot and four times by copying it four times with storm (review C.R. 702.40a). (Note that the magecraft ability word, like all ability words, has no game mechanics inherent to it [C.R. 207.2c].) Here, storm means, in part, "copy it for each spell that was cast before it this turn" (C.R. 702.40a), but since copying a spell normally puts only one copy of a spell on the stack (C.R. 706.10), copying it with storm does one or more individual actions of copying. Similar principles apply to drawing cards (C.R. 121, especially C.R. 121.2; see, e.g., Airborne Aid).
On the other hand, when you gain life with Congregate, Ajani's Pridemate's ability will trigger only once no matter how many creatures you control. Here, "Whenever you gain life, ..." means "Whenever a source causes you to gain life..." (C.R. 119.9), and here there is only one such source, namely Congregate, and the life is gained at the same time because it has the action verb "gain" only once (C.R. 608.2f; compare Congregate with Life Burst).
I cast Grapeshot.
How many magecraft triggers do I get?
I control a Ajani's Pridemate and 4 other creatures.
I cast Congregate.
How many pridemate triggers do I get?
On the other hand, when you gain life with Congregate, Ajani's Pridemate's ability will trigger only once no matter how many creatures you control. Here, "Whenever you gain life, ..." means "Whenever a source causes you to gain life..." (C.R. 119.9), and here there is only one such source, namely Congregate, and the life is gained at the same time because it has the action verb "gain" only once (C.R. 608.2f; compare Congregate with Life Burst).