The youtube api does not have a video get method. So there is no way to directly get just a single video.
I am not sure why you are using the activites.list method.
I would use a search.list and send the channel id. example
request
GET https://www.googleapis.com/youtube/v3/search?part=snippet&channelId=UCWnY0GoKFnFY0oeaEN605Ug&maxResults=50&key={YOUR_API_KEY}
response
{
"kind": "youtube#searchListResponse",
"etag": "\"XI7nbFXulYBIpL0ayR_gDh3eu1k/9HyOjSMVnKv1vm239plm060CuiY\"",
"nextPageToken": "CDIQAA",
"regionCode": "DK",
"pageInfo": {
"totalResults": 71,
"resultsPerPage": 50
},
...
}
It is still returning more then the max 50 rows. You are going to have to use the next page token to get the next page of results.