Many of the nearly two dozen medical marijuana dispensaries already open in Massachusetts are expected to ready pitches to edge into the recreational market, and dozens more could also enter the fray when state regulators start accepting applications on April 1, industry officials say. But whether they, or any others, are actually selling pot by July 1 — when legal sales are allowed to begin — remains an open question, as they prepare to meet both newly cemented state regulations and local guidelines that could differ from town to town.