“Suddenly,

There was

With this angel

A multitude

Of the heavenly host.

They were

Praising God.”

καὶ ἐξαίφνης ἐγένετο σὺν τῷ ἀγγέλῳ πλῆθος στρατιᾶς οὐρανίου αἰνούντων τὸν Θεὸν

Luke said that suddenly (καὶ ἐξαίφνης), a whole choir or a heavenly host of angels came to join this one angel (ἐγένετο σὺν τῷ ἀγγέλῳ πλῆθος στρατιᾶς οὐρανίου).  They were all praising God (αἰνούντων τὸν Θεὸν).  This one angel became a multitude of angels.