This verse implies that the gathering of Israel that took place in connection with the moving of the ark of the covenant to the temple was larger than we might have inferred from 1 Kings 8:1. There the writer speaks of the gathering of the leaders of Israel, and here the language is all the congregation of Israel.
The word translated congregation
(edat) refers to those who are called together by appointment, so the assembly need not be larger than the presence of all the leaders mentioned in the first verse. Nevertheless, given the multitude of sacrifices that were offered, it is possible that the assembly was larger than those men who were in leadership. In fact, this is likely, given the later use of the phrase all Israel.
The main point of this verse is the multitude of sacrifices that were offered to the Lord on this occasion. It is an exaggeration for the text to say, so many sheep and oxen that they could not be counted,
but this is a true statement if it means that such an accounting was impossible under the circumstances.
You might ask what kind of sacrifices were offered, for the law of Moses identified several different kinds of sacrifices, such as sin offerings, trespass offerings, and thank offerings. The text does not tell us, and it may be that more than one kind was offered. Yet the celebratory nature of the occasions in the dedication of the new temple of the Lord suggests to my mind that thanksgiving seems to have been the order of the day. However, if these are the same sacrifices of which the later verses of the chapter speak, then we are told distinctly that they are peace offerings.
5 And King Solomon and all the congregation of Israel, who had assembled before him, were with him before the ark, sacrificing so many sheep and oxen that they could not be counted or numbered.