Based on what we see in other viruses, the practical evolutionary upper limit on R0 for respiratory viruses appears to be about 18.

https://www.vaccinestoday.eu/stories/what-is-r0/

Preliminary data on the Omicron variant indicates an R0 value in the 7 - 10 range. So SARS-CoV-2 might still have a little more room to evolve even higher contagiousness.

https://www.thelancet.com/journals/lanres/article/PIIS2213-2...