In plain words
There is no end to My divine manifestations, scorcher of foes. What I have told you is only a brief indication of the extent of My glory.
Word for word
There is no end of My divine vibhūtis, O scorcher of foes; but this extent of [My] vibhūti has been told by Me only in part.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
न अन्तः अस्ति मम दिव्यानां विभूतीनां विस्तराणां परन्तप । न हि ईश्वरस्य सर्वात्मनः दिव्यानां विभूतीनाम् इयत्ता शक्या वक्तुं ज्ञातुं वा केनचित् । एष तु उद्देशतः एकदेशेन प्रोक्तः विभूतेः विस्तरः मया ॥ ४० ॥
In plain words
"There is no end to my divine powers, no end to how far they reach, Arjuna, scorcher of foes." For the Lord is the Self of everything, and no one can say how much his divine powers come to — no one can even know it. "But this much of that reach I have declared to you" — as a gesture toward it, one portion and no more. || 40 ||
Close to the Sanskrit
"There is no end of the extents of my divine manifestations, O scorcher of foes." For indeed, of the Lord, whose Self is all, of his divine manifestations, the this-muchness is not able to be spoken, or known, by anyone. "But this extent of manifestation has been declared by me" — by way of indication, through one portion only. || 40 ||