What do other scriptures say about total meals per day (1, 2 or more)?

For a Grihastha (Householder), Dharma Shatsras has prescribed to take food two times, in Morning and in Evening. But if one has already taken excess of food in morning, he should not eat second time (in evening).

Apastamba Dharma Sutra, PRASNA II, PATALA 1, KHANDA 1

He shall eat at the two (appointed) times, (morning and evening).

Vishnu Smriti, Section LXVIII

Let him not take a third meal (over and above the two regular meals in the mornings and evenings), nor let him ever take unwholesome food. He must eat neither too early, nor too late, and he must take no food in the evening, after having fully satiated himself in the morning.

Manu Smriti, Chapter 4

न भुञ्जीतोद्धृतस्नेहं नातिसौहित्यमाचरेत् ।

नातिप्रगे नातिसायं न सायं प्रातराशितः ॥ ६२ ॥ He shall not eat anything from which oil has been extract ed; he shall not commit gluttony; he shall not eat very early in the morning, nor very late in the evening; nor in the evening, if he has eaten in the morning.—(62).

And for a Vanaprasthi (Hermit), Dharma Shastra has prescribed one time meal only.

Manu Smriti, Chapter 6

नक्तं चान्नं समश्नीयाद् दिवा वाऽहृत्य शक्तितः ।

चतुर्थकालिको वा स्यात् स्याद् वाऽप्यष्टमकालिकः ॥ १९ ॥ Having collected food to the best of his ability, he should eat it at night, or during the day; or he may do it at every fourth time, or at every eighth time.—(19).

Please check the commentary of Medhatithi for explanation of fourth time or eighth time.

Vishnu Smriti, Section XCV

He must eat at night. He may eat after having fasted entirely for one day, or for two days, or for three days.

Many medical practitioner & gym trainers promote for 6 small meals against 2 full meals for everyone -- common man or bodybuilder. Is it ok?

From the above verses, it is clear that taking 6 small meals in a day is not OK.(This is my understanding)

If twice eating is prescribed then out of breakfast, lunch and dinner, which has to be skipped?

From the above verses, it is clear that one should only eat in Morning and Evening.