Mode length of a sentence
05AB1E, 10 9 7 bytes
-1 byte inspired by Jonathan Allans Jelly answer.
-2 bytes thanks to Kevin Cruijssen!
Input is a list of characters.
aγO0K.M

Try it online!
Commented:
aγO0K.M     implicit input       [&quot;a&quot;,&quot;b&quot;,&quot; &quot;,&quot;c&quot;,&quot;d&quot;,&quot; &quot;,&quot;e&quot;,&quot;.&quot;]

a           is_alpha (vectorizes)[1, 1, 0, 0, 1, 1, 0, 1, 0]
 γ          split into chunks of equal elements
              [[1, 1], [0, 0], [1, 1], [0], [1], [0]]
  O         sum the lists        [2, 0, 2, 0, 1, 0]
   0K       remove 0's
     .M     mode                 2

            implicit output      2

a is implemented as Regex.match?(~r/^[a-zA-Z]+$/, to_string(x)), which should be equivalent to the challenge specification.
I feel like there has to be a shorter way to remove 0s from a list than ʒĀ}.