How many meals is one to have per day?

One is not obligated to eat any specific amount of meals on Chol Hamoed, so long as he does not fast.[1] [Nevertheless, initially it is a Mitzvah for one to have a meal with bread.[2] One is to eat bread twice on Chol Hamoed, once by day and once by night.[3]]

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[1] Admur 529:5 [No Chiyuv of Kavod and Oneg]; 188:10; M”A 188:7; 530:1; M”B 530:1 that even eating fruits suffices, so long as one does not fast

Other opinions: Some Poskim rule that one is obligated to eat two meals on Chol Hamoed which consist of bread, and for this reason if one forget Yaaleh Veyavo he must therefore repeat Birchas Hamazon. [Tashbatz 214 brought in Beis Yosef 188; Nimukeiy Orach Chaim 530:1]

[2] M”A 530:1; Elya Raba 530; P”M 530 A”A 1

The reason: As the Tashbatz rules that one who forgot to recite Yaaleh Veyavo during the first two meals of Chol Hamoed must repeat the Birchas Hamazon. Now, although we do not rule like this opinion, nevertheless the M”A ibid suspects for it initially that one is to eat bread on Chol Hamoed. [Machatzis Hashekel ibid]

[3] M”B 530:1; Kaf Hachaim 530:4; Implication of Tashbatz ibid that one is required to have to have two meals a day on Chol Hamoed; See Nimukeiy Orach Chaim 530:1 that states that perhaps one meal suffices, although he concludes that according to the Tashbatz two meals are required.

The reason: As it is a Mitzvah to honor Chol Hamoed with food and drink, and the main part of a meal is bread. [M”B ibid] The reason that one is to have a meal with bread both by day and by night is because it is similar to Yom Tov on which we also have two meals with bread, one by day and one by night. [Shaar Hatziyon 530:5] However see Nimukeiy Orach Chaim ibid that explains why by Chol Hamoed it would suffice to eat only one meal as the only reason two meals is required on Yom Tov is because of the Kiddush obligation, which is not relevant to Chol Hamoed.