‘The children of the captivity, who were come out of exile, offered burnt-offerings to the God of Israel, twelve bullocks for all Israel, ninety six rams, seventy seven lambs, twelve he-goats for a sin-offering, all this was a burnt-offering to YHWH.'
The personal pronouns here change from ‘we' to ‘they'. This was necessary because here it was all the children of the captivity who participated, not just those who had come with Ezra. It was ‘the children of the captivity who had come out of exile' regardless of when they had come, and this is confirmed by the mention of the offerings being for ‘all Israel'. Clearly the High Priest and the incumbent priests would be equally involved. Thus Ezra's returnees were welcomed by the previous returnees, and all together offered offerings and sacrifices.
There were twelve bullocks for all Israel, one per tribe; and ninety six rams, possibly, but not necessarily, seen as eight per tribe (but note the number of lambs), and seventy-seven lambs. To the people of that day seventy seven would indicate intensified divine perfection. They tended to read into numbers ideas rather than quantity. And these were all offered as whole burnt offerings, that is as offerings of homage and dedication to YHWH which were wholly burnt up. Additional to these were the twelve he-goats sacrificed as a sin-offering, one for each of the tribes of Israel, necessary in order to deal with the sin of Israel so that Israel could be dedicated to God and approach Him in worship. The ‘twelve tribes', would be seen as including the priests and the Levites. All these offerings and sacrifices were seen as ‘a burnt offering to YHWH', symbolic of atonement, dedication and worship.