Question

what is the covalency of carbon in ethene

Answer 1

The covalency of carbon is 2

Answer 2

In ethane, the orbitals are sp3-hybridized orbitals, but single bonds formed between carbon atoms with other hybridizations do occur (e.g. sp2 to sp2). In fact, the carbon atoms in the single bond need not be of the same hybridization .
The covalency is 2.