I'm using Asterisk 11.2.1 and from the extensions.conf sample:
; If extenpatternmatchnew is set (true, yes, etc), then a new algorithm that uses
; a Trie to find the best matching pattern is used. In dialplans
; with more than about 20-40 extensions in a single context, this
; new algorithm can provide a noticeable speedup.
; With 50 extensions, the speedup is 1.32x
; with 88 extensions, the speedup is 2.23x
; with 138 extensions, the speedup is 3.44x
; with 238 extensions, the speedup is 5.8x
; with 438 extensions, the speedup is 10.4x
; With 1000 extensions, the speedup is ~25x
; with 10,000 extensions, the speedup is 374x
; Basically, the new algorithm provides a flat response
; time, no matter the number of extensions.
; By default, the old pattern matcher is used.
; ****This is a new feature! *********************
; The new pattern matcher is for the brave, the bold, and
; the desperate. If you have large dialplans (more than about 50 extensions
; in a context), and/or high call volume, you might consider setting
; this value to "yes" !!
; Please, if you try this out, and are forced to return to the
; old pattern matcher, please report your reasons in a bug report
; on »issues.asterisk.org. We have made good progress in providing
; something compatible with the old matcher; help us finish the job!
; This value can be switched at runtime using the cli command "dialplan set extenpatternma tchnew true"
; or "dialplan set extenpatternmatchnew false", so you can experiment to your hearts conte nt.
So I would expect it can handle a very large number of extensions pretty efficiently.