Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

You can't tap more creatures to convoke Knight-Errant of Eos than is necessary to pay for the spell. In most cases, this means five creatures. However, if there are any additional costs to cast Knight-Errant of Eos, you may use convoke to pay those additional costs as well.

It doesn't matter if the creatures that convoked Knight-Errant of Eos are still on the battlefield or not as the enters-the-battlefield ability is resolving. You'll get credit for them when determining the value of X.

Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.

If a creature you control has a mana ability with in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.

Tapping a multicolored creature using convoke will pay for or one mana of your choice of any of that creature's colors.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.