The chart above is from Axios.com. It shows the Nielsen figures for the viewership of the last four Democratic and Republican convention.
The number of viewers who watched both conventions this year is significantly lower than in previous years. The Democratic convention had 5.2 million fewer viewers than in 2016. The Republican convention had 8.4 million fewer viewers than in 2016.
On the most important night, the last day when the presidential nominees give their acceptance speech, about 800,000 more people watched Biden's speech than watched Trump's speech.