Here in the original manuscript, at the end of a line, Oliver Cowdery initially wrote how after behold. Almost immediately he caught his error and crossed out the intrusive how (there seems to be no change in the level of ink flow for the crossout). It appears that Oliver started to write “yea behold how many mighty miracles have we wrought in this land”, where the normally expected word order would have been have we, not we have. The original manuscript is not extant for that portion of this sentence, but the printer’s manuscript reads “yea behold many mighty miracles we have wrought in this land”. The critical text will accept the reading without the how and with the word order we have.
There is a possibility that the initial occurrence of how here in verse 12 was in anticipation of the how that occurs 18 words later in the text (in verse 13); note, in particular, that the words behold and many are repeated:
Here in this passage, Joseph Smith may have dictated too much of the text at one time, thus leading Oliver to write the how in anticipation of the following “behold how many thousands”. It is also possible that Joseph himself anticipated the following how as he read off the text. For an example of anticipation that seems to indicate Joseph either viewing or dictating at least 20 words at a time, see under Alma 56:41.
Summary: Accept in Alma 26:12 the corrected reading in 𝓞 without the exclamatory how (“yea behold many mighty miracles we have wrought in this land”); the initial how after behold may be the result of anticipating the exclamatory how that does occur in the following sentence: “behold how many thousands of our brethren hath he loosed from the pains of hell” (Alma 26:13).