3 bubbles would have been the answer...
New York has 2 MLB fields that could host a double header daily, and Staten Island to host a day game daily for the East teams. Use Brooklyn as home base.
Chicago has 2 Mlb fields and Milwaukee another. Use Northern Chicago Suburban hotels as home base for the Central teams.
LA has 2 MLB fields...not sure about a third. Plenty of places to use as home base for the West Teams.
But...they couldn't stand to be away from home for 60-75 days. What a bunch of spoiled wimps...
LA you could use the A ball parks in San Bernardino and Rancho Cucamonga. You could do something similar for the Bay Area if you wanted: Oakland, SF, San Jose's A ball park, and even Sacramento if you wanted to expand a bit. Chicago also has a couple Midwest League parks fairly nearby too, especially if you're including Milwaukee.