"Ik zie twintig paarden."
Translation:I see twenty horses.
El2theK is correct. To see is an exception. I guess you could be "seeing twenty horses", but that could be interpreted as you dating twenty horses at the same time. Probably not what you're after ;-)
To see is a non-continuous verb. Have a read through this: https://www.englishclub.com/grammar/verbs-m_vmwct_1.htm
'I see' expresses the standard present. You only use progressive if you really want to emphasise the action, if it is something recurring (e.g. 'I am seeing him every week now') or if you want to refer to something that will happen in the future, for example 'I am seeing him tomorrow' ;-)