Please apply following JSON codes to the lookup column.
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"style": {
"display": "block"
},
"children": [
{
"elmType": "div",
"forEach": "test in @currentField",
"txtContent": "=if(loopIndex('test') < 5, '[$test.lookupValue]', if(loopIndex('test') < 5 && loopIndex('test') < length(@currentField) - 1, ',', if(length(@currentField) > 5, '...', '')))"
}
]
}
Result:
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