speedbar-tag-regroup-maximum-length

speedbar-tag-regroup-maximum-length is a variable defined in `speedbar.el'.
Its value is

10

Documentation:
Maximum length of submenus that are regrouped.
If the regrouping option is used, then if two or more short subgroups
are next to each other, then they are combined until this number of
items is reached.

You can customize this variable.