5th level seems right for your spell
I think it’s appropriate for the spell to be higher than 4th level, due to it summoning many more creatures than, say, conjure minor elemental, which as you say only summons up to four CR 1/2 creatures. However it’s worth noting that to double the number of creatures requires upcasting that spell at 6th level.
From a damage perspective, the guidelines in the Dungeon Master’s Guide recommend 8d6 damage for a 5th level spell that has multiple targets and does half damage on a save. For one which does no damage on a save - or in this case, the equivalent of an attack roll by the summoned creature, which does no damage on a miss - the DMG says to increase this by 25%, so 10d6.
A shadow only has one attack, Strength Drain, which deals 2d6+2 damage. Each shadow might reasonably be expected to last a few rounds or more, thanks to their special traits and multiple damage resistances, and in any case most combats don’t last many more rounds than that, so each Shadow is likely to deal less than 10d6.
But then there’s the secondary effect of Strength Drain, which reduces the target’s Strength score by 1d4, with no save to avoid it. That’s a strong secondary effect - and something I don’t think any spell duplicates. (Enlarge/Reduce and Ray of Enfeeblement are probably closest; both are 2nd level spells that don’t also deal damage.) I’d think this more than makes up for the lesser damage.
There are limiting factors too: the damage is not dealt all at once, making it less powerful; and the caster cannot necessarily summon the maximum number of shadows - that depends on the number of targets who are present, which will most often be fewer than ten. That’s especially true since it also only works on humanoids (thanks kiviiri for reminding me of this). And, as the OP has pointed out, Shadows have disadvantage when in direct sunlight. These restrictions make the spell of situational use, though humanoids are one of the most commonly encountered types of enemies in most campaigns. The limitations remove any doubt that the spell should be 6th level, but I think it’s still powerful enough to be 5th level.
...but to make sure, it needs some more detail
Whether it’s entirely balanced will also depend a bit on some more specific wording, as Carcer has suggested in their comments. When the creatures act, and how long they last, since as currently written the summoned shadows last indefinitely if they or their “owners” are not slain. (This raises all sorts of questions for what happens if an encounter ends without all the targets dead - a likely scenario in the best case for using the spell, a fight with ten or more humanoid opponents!) These are important factors that will alter how much damage they can do before they die, as well as significant narrative effects of the spell, which should also be taken into account. I would suggest clarifying that and presenting a more finished version for further analysis.
And test it at the table! Give it to a player and see if they want to spend a 5th level spell slot on it.