A friend posted on facebook an article by Donald Lopez jr. in the Buddhist magazine tricycle (Winter 2012), beginning with the words:

According to Buddhist doctrine, there can be only one buddha for each historical age. A new buddha appears in the world only when the teachings of the previous buddha have been completely forgotten, with no remnant — a text, a statue, the ruins of a pagoda, or even a reference in a dictionary — remaining. Because the teachings of Gautama Buddha, the historical Buddha — that is, our Buddha — remain present in the world, we have no need for a new buddha.

In the Dhiga Nikaya collection of the Pali canon we find indeed the following statement (ii 225 and iii 114):

Nowhere and at no time is it possible that, in one and the same world-system, two Arahant Buddhas should arise together.

This view is not only expressed in the sutras of the Shravakas, but also in some mahayana texts. Dorje Sherab provides in his commentary on Jigten Sumgön’s Single Intention the following example from a Nyingmapa tantra:^{♦ 1}

I have never seen that there occur
in one world or one continent,
during the period of one eon,
in one country, or on one vajra-seat
two Buddhas who complete the activities.
If there were, it would be impossible and it would contradict the Dharma.

In contrast to that, Jigten Sumgön maintains in the Single Intention (2.15) that “many Buddhas appear continuously in a single Buddha field.” His commentator Rigdzin Chökyi Dragpa argues that by perfecting the familiarisation with the two accumulations of merit and wisdom, the Buddha activity will be equal to the Buddha nature of all sentient beings. At the same time, sentient beings are limitless. Thus, Candrakirti says in his Madhyamakavatara (6.194):^{♦ 2}

Because samsara is
without a first beginning and a final end,
it is called “free from beginning and end.”

Therefore Jigten Sumgön maintains that since Buddha activity equals the Buddha nature of all sentient beings, and since sentient beings are limitless, Buddhas are limitless. This thought is expressed, for instance, in Maitreyanatha’s Uttaratantrashastra (4.62):^{♦ 3}

Therefore, in all the water receptacles,
which are [the minds of] the pure trainees,
immeasurable reflections
of the sun, [which is] the Sugata, appear at once.

Therefore, Rigdzin Chökyi Dragpa concludes, “there come as many Buddhas as there are sentient beings to each realm of the world.” The appearance of one Buddha in one world system does not prevent other Buddhas from appearing there, too. When, for instance, the Buddha was teaching, there often appeared numerous other Buddhas to listen to him and even deliberated the teachings. Our commentaries provide only a few examples, such as the Ratnaketudharani , the Saddharmapundarika Sutra , and the Tathagatasangiti Sutra , but they are certainly numerous.

It is true that only a single Buddha is needed in a world system to reveal the twelve deeds, such as renunciation, ordination, teaching, and entering nirvana, but that doesn’t prevent other Buddhas from “manifesting there as that Buddha’s father, mother, son, retinue, and so forth, even down to showing themselves as non-Buddhists, and they teach the great [qualities],” says Rigdzin Chökyi Dragpa, “since they do not harbor (…) aversion to the Buddha.”

The present vajra statement (2.15) seems to have the same intention as vajra statement (7.15) “all Buddhas dwell in the natural continuum (khams rgyud ) of all sentient beings,” for which see in this blog the article “Where the Buddhas abide.”

Notes
^{1. [Sarvatathagata Cittajñanaguhyarthagarbhavyuha Vajratantrasiddhi Yogagamasamaja Sarvavidyasutra Mahayanabhisamaya Dharmaparyayavivyahasutra , D vol. 97, fol. 201v: ‘jig rten gcig gam gling gcig tu// bskal pa gcig gi gnas skabs la// yul gcig rdo rje gdan gcig tu// mdzad pa mthar phyir sangs rgyas gnyis// ‘byung bar yang dag ngas ma mthong // byung na mi srid chos dang ‘gal//.]↩}

^{2. [Madhyamakavatara by Candrakirti, D vol. 102, fol. 213v.]↩}

^{3. [D vol. 123, fol. 70r.]↩}