qurgh 2025:

Is the word measurable or quantifiable, but not necessarily countable?

vItlh means "a high amount of that thing".
law' does not work with these words.

Is the word countable?

vItlh means "is an unexpectedly large number of those things".
law' means "is many of that thing".

Okrand - qepHom 2017:

It's used for things that are measurable or quantifiable but not necessarily countable. So you'd use it for things like speed or distance or the price of something: Do vItlh "high velocity," chuq vItlh "great range, great distance." You wouldn't use law' in these instances.

For things you can count (like people) (as opposed to things you can measure using units of measure, like length or speed), you'd use law' almost all the time. But you'd use vItlh if the number of whatever it is you're talking about is higher than normal or greater than before or greater than what was expected.

So if you were referring to a lot of warriors, you'd most likely say SuvwI'pu' law'. But if the high number of warriors is somehow important, if it's the point you're trying to make and not just an added fact, you'd use vItlh. You'd use it if you're not saying merely that there are a lot of warriors, but that the quantity of warriors is particularly high (higher than usual, higher than expected, higher than some other similar group of warriors, etc.). So it sounds like L'Rell used vItlh correctly in referring to the large number of people who died in battle (more than expected, more than before, etc.).

Note that the exact number of warriors (or whatever) doesn't matter (and doesn't have to be stated). The idea is that the high number of warriors you're talking about is somehow noteworthy (the number of warriors who died is greater than the norm for this sort of military unit, for example).