Just a note.....
Some manuscripts say 70 were sent while others say 72 were sent. According to some of the commentaries I looked at, there is good support for both readings. Below is one such discussion found in the Holman Apologetics Commentary on the Bible.
The Lord appointed 70 others (10:1, 17): Were 70 or 72 sent out on mission? A complex textual issue stands behind this question, with the same difference appearing in two verses, 10:1 and 17. The external evidence on verse 1 is almost evenly divided: 70 is found in ℵ, A, C, L, W, Δ, Θ, Ψ, family 1, family 13, Byz, some Itala, Coptic, and some Syriac; 72 is found in P75, B, D, some Itala, Vulgate, and some Syriac. P45 is fragmentary at 10:1, although Metzger (1958-59, 299) held that it reads 70.
There has been much discussion about the possibility that the number is intended to bear symbolic meaning. If 70 is the original reading, it may trace its meaning to several antecedents, including Moses’ 70 elders (Exod 24:1, 9; Num 11:16-17, 24-25); the 70 nations of the earth (Genesis 10-11; but note that the LXX lists 72 here); a rabbinic tradition holding that Moses’ commandments were heard in 70 languages (b. Shabbat 88b, though this is late); and the Sanhedrin’s 70 members, excluding the high priest (m. Sanh. 1.5-6).
If the original reading is instead 72, it may have been influenced by the local councils of 72 (m. Zevahim 1.3; m. Yadayim 3.5; 4.2); the 72 translators of the LXX (Letter of Aristeas 46-50); the LXX tradition of the 72 nations in Genesis 10-11 (Marshall 1978, 415; Ernst 1977, 331); or the 72 princes and kings in the world (3 Enoch 17.8; 18.2-3; 30.2).
These listings suggest 70 is of greater canonical significance than 72 (also Rengstorf, TDNT 2:634). Thus, a copyist looking for significance in the number would be more likely to change 72 to 70 than vice versa. However, if Luke genuinely intended to make an OT allusion, then 70 is more likely to have been original.
But does Luke intend his readers to interpret the number symbolically? The two OT events that scholars have most strongly suggested as conceptual background for a symbolic use are not compelling. First, against the suggestion that the LXX listing of 72 nations at Genesis 10-11 is background: the mission of Luke 10 does not reflect the universal context of Genesis 10-11. Though Jesus did not explicitly restrict the mission to Israel here (as he did in the Matthean mission with the Twelve; Matt 10:5-6), it is nevertheless clear in Luke’s writings that an active Gentile mission developed only later, as recounted in the book of Acts (Schmid 1960, 184; Fitzmyer 1985, 846). Thus it is hard to see a connection between Luke 10 and the Table of Nations in Genesis. Second, against the suggestion that Moses’ selection of 70 elders is background, Luke does not pick up and develop Moses’ selection of 70 anywhere in Acts, which would seem an ideal location for him to develop this theme given the church-planting focus of the early church. In light of the weakness of both of these suggested backgrounds for symbolism, it is unlikely that Luke attached symbolic meaning to the number of people sent on mission.
Though we cannot establish a definitive choice between the variants, the absence of a clear symbolic reference throws us back on the practice of looking for the more difficult reading. Given the more obvious canonical significance of 70, and given that copyists would be unlikely to convert the reading from the more obvious to the less obvious, the more difficult reading seems to be 72. For this reason, 72 is slightly more likely to be original.
Michael Wilkins et al., Holman Apologetics Commentary on the Bible – Gospels to Acts, ed. Jeremy Royal Howard, (Nashville, TN: Broadman & Holman, 2013), WORDsearch CROSS e-book, 419.
The headings, if I remember correctly, are a common shared heading (I think from the KJV), therefore regardless the translation you are reading, you get the same text for the headings.
So they (the headings) could very well come from a version (KJV ?) that used a manuscript which reads "70 were sent" whereas the bible translation you may be viewing may have come from a manuscript that read "72 were sent".
Bakers New Testament Commentary and The Bible Knowledge Commentary, both resources available for e-Sword, also discuss this topic (and there are most likely other resources as well, these are just two I found quickly looking through my library).