With the Imperial Guard, taking one of any type of unit *heavy tanks, (viz a viz Leman Russes), chimeras (including but not limited to chimeltas), foot-sloggers, Hellhound variants, Valkyries, etc,* is an easy way to get that unit targeted. If you can use your list and the terrain to make that less likely, by all means limit it to one Valkyrie. If you are really worried that your space helicopter is going to get sniped no matter where you deploy it (plausible), double up. The survivability of any single unit of a set of similar units (ex; space helicopter army) goes up at least at the rate than its cost does.

Let's run a theoretical, inherently generalized and non-specific example, that does handily show the potential benefit of taking two Valkyries (or any unit). This probability experiment assumes that there is a constant ratio of anti-armor units to Valkyries.

Let's assign an arbitrary probability that a Valkarie survives a round; 1/6. That's pretty bad, but it'll get better.

So let's imagine that we take two Valks. The probability that exactly one survives is 10/36, approximately 1/3. The probability that both survive is 1/36. Now the probability that **a** Valkyrie survives a single round is 11/36, a little less than 1/3 still. That's a lot better than one. In fact, it's almost exactly double the probability. You paid for twice the effectiveness.

Now let's imagine that we take three Valks. The probability that **none** will survive is 125/216, or about 5/9. The probability that exactly one will survive is 75/216, or about 1/3. The probability that two will survive is 15/216 or about 1/18, and the probability that all three would survive is 1/216. The probability that **a** Valkyrie will survive is 91/216, slightly more than 5/12.

Now, this seems fairly linear, until we take a look again at the assumption we made at the beginning; a constant ratio of anti-armor to Valkyries (in order to produce a constant survivability). If your opponent devotes the same amount of anti-armor to two or even three Valkyries, the rest of your tanks will run rampant over his army. If he does not keep devoting so many anti-armor units to your Valkyries (because he's worried about your tanks too or because he simply doesn't have that much anti-armor), then the probability of them surviving goes up further, more than a linear cost-to-survivability ratio. That is a good thing.

This method is reasonably expensive, though, and is termed "overloading" because you have overloaded the opponent's capacity to take down armor effectively. However, you were already using this strategy in the form of your chimeras.

EDIT: Your opponent's cost in points of anti-armor goes up proportionally with your cost in Valkyries. That quite quickly adds up to some large costs on his part.

Alright... main point of that wall of text... more Valkyries = more survivability for themselves

or other armor.