[Grammar] they need only say that many others think so

[wotcha]

.....they need only say that many others think so.......

Is this sentence grammatically correct?

 

[bhaisahab]

.....they need only say that many others think so.......

Is this sentence grammatically correct?

Yes, it is, if you start it with a capital letter and end it with a full stop. Otherwise it is not a sentence.

 

[PlacidRan]

Although not as common, yes, it is grammatically correct.
When it comes to ellipsis, do not use more than three points, or four (when ending a sentence). However, three is also fine.