Something you probably already suspect: It happened many times:
The presence of these chunks, making up 2%, on average, of the genome of anyone with roots in Europe, Asia, Australia or the Americas, pointed to a single period of intermingling – probably 50,000 to 60,000 years ago – not long after Homo sapiens emerged from Africa.
But that simple story was complicated by the discovery that people in East Asia have up to 20% higher Neanderthal ancestry than present-day Europeans.
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Evidence for multiple matings already exists, in the form of a 40,000 year old human fossil from Romania with a recent Neanderthal ancestor, whose Neanderthal DNA nonetheless did not become part of modern-day human genomes.Dyani Lewis, “Humans and Neanderthals were frequent lovers, genetics reveals” at Cosmos Magazine
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See also: Neanderthal Man: The long-lost relative turns up again, this time with documents
and
Was Neanderthal man fully human? The role racism played in assessing the evidence