Here's what Douglas Stewart has to say in his commentary:
28:29–30 These two verses indicate the two special purposes of the breastpiece: representation and divination. With regard to representation, the high priest (Aaron stood for himself and all his successors) “will bear the names of the sons of Israel over his heart.” Why? “As a continuing memorial before the LORD,” that is, as a symbol of the fact that whatever he did, he did as the people’s representative, and his actions would have the same essential effect they would have if all of them, one by one, had done the same thing. The high priest symbolized all Israel.
Excursus: The Urim and Thummim
As regards divination (seeking the will of God by some sort of physical action), the breastpiece’s pocket was the holding place for the divination stones, the Urim and Thummim (“lights” and “darks”424). How were these used? There are three strong possibilities: (1) A number of light and dark stones were contained in the pouch. When, in faith and with prayer, the high priest asked God questions, he would then pull out stones one by one to give answers, a light stone perhaps indicating yes and a dark one indicating no (or vice versa). (2) Only two large stones were in the pouch. Each time the high priest had a question for God on behalf of the people, he reached in and found one. Its color told him, as above, the answer. Then he replaced the stone and reached again for a new answer. This seems unlikely since it would have been all too easy for the priest to have a fairly good sense of which of the two stones was where in the pouch. (3) Two or more stones were in the pouch, shaped something like dice, with alternating sides or faces colored light and dark. These were pulled from the pouch, “thrown” as dice are thrown, and examined to see which colors came “up.”425
We have no indication from biblical material that allows us to sort among these options and understand what the Urim and Thummim looked like and how they actually were employed physically. What we do know is that God sometimes chose to reveal his will in this manner rather than by speaking directly to the people. No less an important decision than the choosing of a replacement for Judas for the inner core of the church’s apostolic witness was reached in a similar manner in the New Covenant (Acts 1:26), suggesting that God may honor a nonverbal means of choice or discernment of his will from among options or in response to questions posed. It should be obvious to the reader of the Pentateuch and the Bible in general, however, that God did not and does not normally use “divination” methods as his primary mode of revelation.
The term “over his heart” is used twice in these verses (once in the form “over Aaron’s heart”)—referring in turn to the position of the names of the Israelite tribes as well as to the Urim and Thummim stones used to discern the will of God. The Bible does not teach that the heart was the center of the emotions or of the spirit or of the mind, but all these concepts were alive and well in the culture in which the Bible was revealed and were part of the idiomatic language and conceptual framework reflected in the Bible’s language (much like the expressions “learn it by heart” or “I haven’t the heart for …” in today’s English). Accordingly, having the breastpiece and its contents “over Aaron’s heart” helped Israelites understand the value of these objects and the corresponding value to God of his covenant people and of his desire to reveal himself to them.
Theologically, the Urim and Thummim represented something on the order of last resort appeals to God for guidance—not individual guidance but national guidance on matters that would require the agreement and concerted effort of the whole people.426 The people’s first resort was supposed to be obedience to the written covenant since the written covenant constituted the most basic or foundational guidance, generally and perpetually applicable, that they possessed. The second resort would be to listen for direct divine guidance through the word of God from a prophet, something that God occasionally, but not necessarily regularly, gave them. The third resort would be prayer, and Urim and Thummim use fall under this category. With prayer, seeking to understand how best to take a national direction of some sort, the Urim and Thummim would be drawn from the breastpiece pouch and examined for God’s answer to the people’s prayer. It may be assumed that they understood or should have understood that if they had faithfully tried the first two resorts and received no word from the Lord, they might in faith try the third resort, not as a matter of right but as allowed by and guided by an orthodox high priest ministering properly “before the Lord.” It may also be assumed that since the Israelites were drawn more often to heterodoxy than orthodoxy and usually preferred idol worship to keeping Yahweh’s covenant, their use of the Urim and Thummim would most often have failed—and that they and/or the high priest at any given time may not have known the difference, blinded as they were by their disobedience. The Urim and Thummim were mechanical devices of divination that had validity in certain limited contexts and only as God chose to guide the hand of the high priest in response to faithful prayer from an obedient people. In themselves they were simply colored stones that possessed no intrinsic powers.
Douglas K. Stuart, vol. 2, Exodus, The New American Commentary (Nashville: Broadman & Holman Publishers, 2006), 611-13.
It would seem, grammatically, that there are always at least three urim and at least three thummim. This is supported by the fact that they always occur in the plural form, rather than the singular or dual forms. So I suppose we are looking for at least six stones in the breastpiece.