Does Alpha Strike require multiple enemy units at a battlefield?

No, Alpha Strike does not require multiple enemy units at a battlefield.

According to RiftJudge FAQ #9467, you are not required to target all enemy units at a battlefield, nor are you required to target multiple units. Because the card allows you to "split" damage, you may choose any number of available enemy targets, including just one.

As noted in the FAQ, if you choose to target any enemy units that have the Deflect keyword, you must pay the Deflect cost for each of those units as an additional cost to play the spell.