Myr Battlesphere

When Myr Battlesphere enters, create four 1/1 colorless Myr artifact creature tokens.
Whenever Myr Battlesphere attacks, you may tap X untapped Myr you control. If you do, Myr Battlesphere gets +X/+0 until end of turn and deals X damage to the player or planeswalker it's attacking.

Modern Horizons 3 Commander

When Myr Battlesphere enters the battlefield, create four 1/1 colorless Myr artifact creature tokens.
Whenever Myr Battlesphere attacks, you may tap X untapped Myr you control. If you do, Myr Battlesphere gets +X/+0 until end of turn and deals X damage to the player or planeswalker it's attacking.

March of the Machine Commander

Phyrexia: All Will Be One Commander

The Brothers' War Commander

Commander 2021

As the last ability resolves, you can tap untapped Myr you control even if Myr Battlesphere is no longer on the battlefield by then. If that has happened, Myr Battlesphere won't be able to get the +X/+0 bonus, but it will still deal X damage to the appropriate player or planeswalker.

You can tap any untapped Myr you control as the last ability resolves, not just the Myr tokens you created with the first ability. This includes Myr that haven't been under your control since your most recent turn began.

You choose the value for X as the last ability resolves. You can't choose a value for X that's greater than the number of untapped Myr you control.