20 results on the first page. But usually there are several pages.

When you make a Requisition like this you get much more than 20 results.
https://api.themoviedb.org/3/movie/upcoming?api_key=[MY_KEY]&language=en-US&page=1
There are 20 results per page, but in this case we have 25 pages totaling 497 results.
See the bottom of the first page for this information:

  ],
  "page": 1,
  "total_results": 497,
  "dates": {
    "maximum": "2019-11-16",
    "minimum": "2019-10-20"
  },
  "total_pages": 25
}

Specifically in Top Rated see the amount that is at the beginning of the answer:
https://api.themoviedb.org/3/movie/top_rated?api_key=[MY_KEY]&language=en-US&page=1
6541 results in 328 pages

{
  "page": 1,
  "total_results": 6541,
  "total_pages": 328,
  "results": [
    {
      "popularity": 22.247,
      "vote_count": 2134,
      "video": false,
      "poster_path": "/2CAL2433ZeIihfX1Hb2139CX0pW.jpg",
      "id": 19404,
      "adult": false,
      "backdrop_path": "/pVGzV02qmHVoKx9ehBNy7m2u5fs.jpg",
      "original_language": "hi",
      "original_title": "दिलवाले दुल्हनिया ले जायेंगे",
      "genre_ids": [
        35,
        18,
        10749