Trulli - Button - Kubica - Kovaleinen - Vettel stll stands as 5, though.
I'm not convinced that your 7 stands,
Wombcat , because the Monza Gorilla wasn't a registered entrant for the end of 1978 or the beginning of the 1979 season...
You're right, he missed the last two races in 1978, and only entered the last three races in 1979, and he drove again in 1980 in Holland and Italy.
So it would be:
After Austria 1977 there was Watson, Nilsson, Brambilla, Mass, Laffite and Jones as one-time winners. So that was 6.
Nilsson quit after 1977, back to 5.
Monaco 1978 was won by Depailler, so then there were 6 again.
Brambilla didn't enter in tne US, so back to 5.
Canada 1978 Villeneuve won, and made it 6
1979 started with Watson, Mass, Laffite, Villeneuve, Depailler and Jones as one-time winners.
But that was very briefly, since Laffite won the next race: Brazil 1979, so then there were 5 again.
Until 2 races later (South Africa) where Villeneuve won his 2nd race and it went back to 4.
Depailler won his 2nd in Spain, back to 3. .
Jabouille won his first in France, then it was 4 again.
Until Jones won his 2nd race in Germany, down to 3.
Brambilla entered in Italy and it went up to 4.
At the start of 1980 you had Watson, Mass, Jabouille as one-race winners (3).
Arnoux made it 4 in Brazil, but again down to 3 in South Africa.
Then in the next two races Piquet and Pironi joined the group, so it went up to 5 again.
Until Jabouille won his 2nd in Austria and it was down to 4.
Brambilla re-enterd in Holand and Piquet won that one, so during that race it was 5.
So it looks like 6 is the maximum and this was:
AU 1977-JP 1977 (Watson, Nilsson, Brambilla, Mass, Laffite and Jones)
MON 1978-IT 1978 (Watson, Brambilla, Mass, Laffite, Jones and Depailler)
CA 1978-AR 1979 (Watson, Mass, Laffite, Jones, Depailler and Villeneuve) this was only from the finish of Canada, till the finish of Argentina, so only 1 race, but since it was during the winter, it was quite a long stretch
Altough in 1982 Mass rejoined and that year spurred 5 new race winners, who only won once that season: Tambay, Patrese, De Angelis, Rosberg, Alboreto. Since Mass and Pironi were the only one-race winners at the start of the season, and Pironi won his 2nd in Imola and Mass left again before most of those 5 won their races, there were never 6 one-time race winners racing.
Feel free to correct