Rupa Goswami quotes the following verse from the sixth canto of the भागवतम् in भक्तिरसामृत सिंधु 1.2.52:

मुक्तानाम् अपि सिद्धानां नारायणपरायणः|
सुदुर्लभः प्रशान्तात्मा कोटीषु अपि महामुनेः||
muktānām api siddhānāṁ nārāyaṇa-parāyaṇaḥ
su-durlabhaḥ praśāntātmā koṭiṣv api mahā-mune

Jiva Goswami helps us translate this verse by defining the terms in it. According to him,
muktānām = prakṛta-śarire-sthatve’pi tad-abhimāna-śūnyānām = among those who, despite being situated in the material body, are devoid of identification with it.
siddhānāṁ = prāpta-sālokyādīnām ca = and among those who attained sālokya or other muktis.

nārāyaṇa-parāyaṇaḥ = nārāyaṇa-sevā-mātra-kāṅkṣī sudurlabhaḥ = one who exclusively desires only the service of Nārāyaṇa is exceedingly rare.

So putting it all together, the verse means:

Among millions of those who are devoid of identification with the material body, and those who have achieved sālokya or other muktis, one with a peaceful mind who exclusively desires only the service of Nārāyaṇa is exceedingly rare.

The uttama bhakta is rare not because there are few bhaktas. As the verse points out, the uttama bhakta is rare even among those who achieved sālokya or other muktis. That is, most devotees who come to bhakti come to satisfy some demand, and this demand is for something other than bhakti, and therefore they are not uttama bhaktas (see this for more).

Probably the majority of modern bhaktas come for relief from misery or suffering. Or they may be looking for companionship or love and get captured by aggressive groups (see https://en.wikipedia.org/wiki/Love_bombing). Here is also a TED-Ed lesson on this topic: https://ed.ted.com/lessons/why-do-people-join-cults-janja-lalich.

It also follows from the above verse that there are many devotees in the spiritual world who are not there out of a desire to exclusively serve Nārāyaṇa. Being in the spiritual world is also a type of service, but devotees who desired sālokya desired it because the spiritual world is a nice place to live devoid of suffering! Without doing bhakti, one cannot go to the spiritual world, but one does not have to be an uttama bhakta to get there. Such devotees do their puja or worship regularly even in the spiritual world, and are devotees, but they desire something else in addition to bhakti.

We can see a modern version of this¹– many people are buying flats in Vṛndāvan these days. Their idea is to die there- so many retired people are moving there to die – and there are many statements in the scriptures that exalt dying in Vṛndāvan because it gives prema. Or there are statements that one can take three dips in Radha-kunda and get prema.

These statements from the scriptures are true of course, but one only gets the fruit- prema- if one actually wants it from these activities. Most people dont want uttama bhakti; actually the majority dont even know what uttama-bhakti is. So how is one to get the fruit of dying in Vṛndāvan? We get what we want- and most people want freedom from misery or find some sense of belonging. Therefore it is critical to understand what uttama-bhakti is.

1. भक्ति रसामृत सिंधु lectures, Bhakti Tirtha II, Shri Satyanarayana dasa Babaji, Jiva Institute, Vrindavan. 2017. ↩