Do you have to target all any units at battlefields when you play alpha strike?

No, you do not have to target all enemy units at battlefields when you play Alpha Strike.

According to Rule 355.13, if a card specifies that a player chooses "any number" or "up to" some number of objects to be affected, they may choose any number of available targets, including zero. While Alpha Strike does not explicitly use the words "any number," the instruction to "split" damage among enemy units implies that you choose which units receive that damage.

Furthermore, Rule 355.14.a states that if a card specifies an amount of damage may be split among some number of units, each unit chosen is Targeted. You are only required to target the enemy units you intend to assign damage to. You are not required to target every enemy unit present at the battlefields.

Important Note on Deflect: 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 Alpha Strike (as detailed in RiftJudge FAQ #8721). If you cannot pay the Deflect cost for a specific unit, you cannot choose that unit as a target for the damage.