Probably just about anything you want
Bounded accuracy makes this a lot more feasible than you might expect. Unlike 3.5 where a 20th level character could utterly destroy low-level nothings with general impunity, that's not the case with 5e.
First of all, let's do some math to figure out how many stirges could actually attack this creature. I'm making the following assumptions:
- X = Square root of the number of squares occupied by the creature on a 2-d plane. So X=1 for a medium creature and X=3 for a huge creature.
- From your question, I'm assuming 4 stirges can occupy a 5'x5'x5' cube for battle purposes.
- Any creature being considered is assumed to occupy a cube. Thus, a medium creature occupies a 5'x5'x5' cube.
From these assumptions, we generate the following equation to determine how many stirges can attack a creature:
- Number of squares around creature = [(X+2)*2 + 2X] * X
- Number of squares above creature = (X+2)^2
- Total is multiple by 4
From this we find the following:
- Number of squares around creature = 8
- Number of squares above creature = 9
- 17 squares * 4 = 68 stirges
- Number of squares around creature = 24
- Number of squares above creature = 16
- 40 squares * 4 = 160 stirges
- Number of squares around creature = 48
- Number of squares above creature = 25
- 73 squares * 4 = 292 stirges
- Number of squares around creature = 80
- Number of squares above creature = 36
- 116 squares * 4 = 464 stirges
So what can you kill with 464 stirges? Depends on how often they'll hit, but with a +5 to hit coupled with bounded accuracy, you're looking at quite a few hits.
I did a skim of gargantuan creatures and to exemplify my point, I picked:
With an AC of 22 and 481 hit points, this is a CR23 monster that notably does not have any resistance to non-magical attacks.
The stirges can only hit with a 17 or better. So on a roll of 17, 18, or 19 they'll deal an average of 5.5 damage. On a roll of 20, they'll deal an average of 8 damage. Thus, every 20 stirges that attack will deal, on average, 24.5 damage; this means the average damage per stirge is 1.225 damage.
With 464 stirges attacking the Ancient Blue Dragon, in one round they will deal approximately 568.4 damage.
At 18% more than the dragon's max hit points, you could reasonably say that dragon is definitely dead.
Maybe we can do better with something that has resistances, so lets try:
AC of 20, 297 hit points, resistance to non-magical attacks.
The stirges hit on a 15 or better, but damage dealt is halved. On a roll of 15, 16, 17, 18, or 19 they'll deal an average of 2.25 damage. On a roll of 20, this will improve to 4 damage. So for every 20 stirges that attack, you're looking at 15.25 damage and average per stirge damage of 0.7625.
This leads to a 1 round damage output of 353.8 damage, about 20% more than the max. So this is also reliably killed.
If you want a gargantuan creature dead, and it's not immune to normal weapons, 464 stirges should do the trick.