Q1:
A few nouns in English have identical singular and plural forms. Sheep is an example. If you want to use sheep as singular noun you have to use "the sheep is grazing"
Q2.
Ten dozens pencils were sold
Ten dozen pencils were sold
When the nouns of numbers are followed immediately by of, their plural forms are used. I have dozens of pencils.
Q3.
Five hundred students assembled there
Five hundreds students assembled there
Hundreds can be used if there is no number or quantity before it
Q4.
Credentials was presented by the envoy
Credentials were presented by the envoy
Credentials is the plural of the word credential
Q5.
The noun riches is plural only. The plural form of riches is also riches.
