In substrate-based chains, there is no limit on how many nominators can nominate a single validator, however there is a maximum amount of nominators that can back a single validator in the active set. Beyond that limit, the validator is considered oversubscribed and it will only be elected using the maximum allowed number of nominators. Nominators with a higher stake take precedence, so nominators with a lower stake than the lowest stake nominator of maximum allowed number of nominators are disregarded and will not earn any awards during that era.
The limit of maximum allowed nominators are:
Polkadot, Kusama & Edgeware - 128 nominators
Hydra - 64 nominators