"They see six elephants."
अनुवाद:वे छह हाथी देखते हैं।
Placing of 'को' correct in this place but it's not used in such sentences. Generally, 'को' is not explicitly used in present indifinite tense . (वे हाथी देखते है) . But in present perfect tense . (उन्होनें हाथी को देखा) . Remember in 'Present perfect tense' work is perfectly done in present. In present idifinite, completation work is in indifinite. Note that 'ने' is also not used along with 'को' in present indifinite tense .