I tried my 1st ones and it seemed like a success,

However what confuses me is.
I wouldnt think of myself to have good ab strength(apart from doing sit-ups for a mediumish period i dont do anything with them), as its the 1st time i did these crunches, I tried doing them the difficult way with my hands behind my head touching my opposite shoulders. I did 15 and i i didnt find it hard(i didnt flex abs becuase it was 1st time doing them and tbh i wasnt sure if i was flexing them or not) I also stayed up for 5 seconds
So i did another 3 sets all of 15 and i`m not getting dat tired, i took roughly 5sec brake between each set
Should i do more per set? rather than keep it 15 and end up doing 5sets to make sure muscle is overloaded?
Since i could have continued doing a few more i`m under the impression i just wasted my time as i didnt overload muscles.
So i was looking at 30 crunches for 2-3sets, or mybe 25 crunches 3 sets.
Or could it be i didnt go up high enough and hence found it easy?