If we set talent dilution aside (which I am not sure is an issue these days to be honest). I figure they could all go the 34-36 teams. The NFL could go to 40 if they include Europe. This is what I see the 4 major leagues being able to do:
The NBA could put teams in the following cities: Seattle, Vegas, Vancouver, San Diego, Louisville, and Montreal. Figure Memphis and New Orleans are relo candidates so thats 34 teams.
MLB could put teams in Montreal, Portland, Vegas, Nashville, and Charlotte. Figure 2 of those spots can be taken by Tampa and Oakland. So they could go to 34. Or 36 if you take those 5 cities and add Mexico City.
The NFL could put teams in San Antonio, St Louis, Mexico City, Toronto, plus 4 in Europe. Only Jacksonville is a relo candidate. There is probably at least one more North American city I am missing so that's 40 they could do.
The NHL could put teams in QC, Hamilton, Portland, Salt Lake City, and Houston. Arizona is an obvious relo candidate. Maybe Florida if last years attendance spike isn't sustained. so 36 is a reasonable number for them,