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There are some instances where an atom can hold more than eight valence electrons, it is called an expanded valence shell/expanded octet. One example of this would be PCL5. I don't think we need to know about this for the basics of Lewis structures though.
Elements in the third period and after generally have exceptions to the octet rule where they can take on more electrons than eight. However, prior to that, H and He will not follow the octet rule, but rather the "duet rule" because they will only try to fill up their s orbital. Boron also sometimes forms compounds where it is electron deficient, however compounds where it does obtain the octet will be more favorable.
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