What is the maximum number of columns allowed on a table in MySQL MEMORY storage engine?
From This MySQL link


  There is a hard limit of 4096 columns per table, but the effective
  maximum may be less for a given table. The exact limit depends on
  several interacting factors.
  
  
  Every table (regardless of storage engine) has a maximum row size of 65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.
  
  
  ...


These are engine-independent MySQL limits. As you say the MEMORY engine specific manual pages do not seem to specify any additional restrictions for size or count.