𝓞 is extant for this part of the text and reads supficeth, Oliver Cowdery’s miswriting of sufficeth. Three other times in the manuscripts Oliver miswrote ff as pf:
Regarding the last example, Oliver Cowdery sometimes spelled profit with two f ’s, as proffit, which would then make the miswriting propfit possible:
The problem with the miswriting supfiseth here in Alma 56:5 is that it led Oliver Cowdery to misread that word as supposeth when he copied the text from 𝓞 into 𝓟. The 1830 edition followed this secondary reading, as did the second edition (1837) and the first two British editions (1841 and 1849). For the 1840 edition, Joseph Smith replaced the secondary supposeth with the correct sufficeth. It seems doubtful that Joseph referred to 𝓞 in making this emendation. It is obvious that sufficeth works better, although one can accept supposeth by assuming some kind of semantic ellipsis of a modal verb (as if Helaman meant to write “therefore it supposeth me that I should tell you that …”). The first printing of the 1852 LDS edition followed the 1849 reading, supposeth (which ultimately derives from 𝓟), but the second printing restored the correct sufficeth (by reference to the 1840 edition). The LDS editions have continued with sufficeth, but the 1908 RLDS edition restored supposeth to the RLDS text because 𝓟 reads supposeth.
Elsewhere in the text, the complement for the expletive it in the phrase “it sufficeth me” is an infinitive clause (usually with the verb say):
On the other hand, with “it supposeth me” the it is always complemented by a that-clause:
Thus here in Alma 56:5, the it is exceptionally complemented by a that-clause (“it sufficeth me that I tell you that …”, not “it sufficeth me to tell you that …”). Thus from a syntactic point of view (but not a semantic one), the verb suppose works better (“it supposeth me that I tell you that …”). But there are examples of existential suffice that permit other types of complements besides the infinitive clause:
In the first case, we have a conditional if-clause, in the second a that-clause. Thus the use of the that-clause here in Alma 56:5 for “it sufficeth me” is possible, and the critical text will maintain it.
Summary: Maintain in Alma 56:5 the reading of the original manuscript, “it sufficeth me that I tell you that two thousand of these young men hath taken their weapons of war”.