I think I understood your question.

In case of ionic bonding, an electron is "lost" by the one and gained by the other. So, it depends on how badly electrons are wanted or not wanted - quantified by electronegativity. More the difference in electronegativity, higher the ionic nature of the bond.

In case of covalent bonding, no one wants electrons to be "lost". So, instead of absolute "give and take", they share. This is what happens in case of $\ce{CH4}$ as well.

Finally, the limit that you speak of does not appear as "how many electrons", but as "what difference in electronegativity". In practice, no bond is perfectly ionic and has some covalent character, but the ionic character is directly related to the difference in electronegativity.