Well Buddha himself did say he was enlightened of course. But you don’t need to see it all the way to the end goal to see results of following the path. Many of the Buddhist teachings are about practical things you can put into place right away and you can judge them by their effects on your life. Buddha taught a path to cessation of dukkha, unsatisfactoriness. He then invited us to follow the same path he did.

So when Buddha became enlightened, according to his life story, he had no proof that enlightenment was possible. He just saw the nature of dukkha, unsatisfactoriness and he was looking for a path to deal with it. He went to the best teachers of his day, and they taught him various ways to calm his mind and to enter refined states of pure bliss with not a trace of suffering or anxiety, at all, and also states even more refined than that. He learnt to use these meditations and then asked his teachers if that was all that they had to teach him, and they said “yes, you’ve realized our teachings”. They also asked him if he’d teach himself or even lead their movement. But having found out what it was they taught from his experience, from putting it into effect, he decided this was not what he was looking for. It was based on conditions and he could see that anything generated based on conditions would be dependent on them and so dukkha.

So - he wasn’t saying there was anything wrong in such practices. Just that it wasn’t what he was looking for. He gave teachings to help pracitioners find happiness in all forms including happiness that’s not lasting, and to be free from suffering temporarily. To find happiness and to be free from suffering temporarily is good!

Dukkha doesn’t mean suffering or even unsatisfactory really, as these states can be highly satisfactory of course. Even worldly pleasures and achievements often are very satisfactory. But he taught that one can come to see that these are not the end of all unsatisfactoriness for all time, so it is still dukkha, conditioned, dependent on conditions and so subject to change and unsatisfactoriness.

So - this much we can all see for ourselves. So on the path, we are following it much as the Buddha did. He had no example to show that Nirvana is possible. We do, with his life story as an example. That shows it - but doesn’t prove that it is possible, either. Just as the Buddha did, followers of the Buddhist path also set out on this process of exploration and discovery, with no guarantees. So it is based on a faith, much as Buddha had, that there is some way to cessation of dukkha.

As for showing results - well when Buddha realized Nirvana he continued as an ordinary person to all appearances; he didn’t suddenly vanish or turn into a celestial being. He said to his followers that he had realized nirvana - yet he got old, he got sick, eventually he died. So if you encounter someone who has realized Nirvana they would be the same as anyone else. The first person Buddha met after he became enlightened didn’t believe him when he said he was enlightened.

So - it’s not so much that there is a prohibition on a Buddha - that they can’t say that they are enlightened - because Buddha did say he was enlightened. It’s rather, that it often doesn’t help. If you meet a Buddha, their only reason for telling you that they are Buddha is if it will help you and others to say so. If not, they won’t, because they don’t have anything to prove to anyone else.

And as for how many are Buddha - well we can’t really say. In some traditions they talk about vast numbers of Buddhas outnumbering those who are still unenlightened. In the Therevadhan traditions, Buddhas enter paranirvana when they die. When asked what that means, Buddha just didn’t answer, saying it was not helpful to answer such questions. He gave no answer to whether a Buddha exists after entering paranirvana or doesn’t, or both, or neither. All that was getting immersed in concepts and a thicket of views.

In the Tibetan and other Mahayana traditions, that’s true of Wheel turning Buddhas (the ones who teach the dharma when there are no teachings of that type available yet, or they have been totally forgotten). But other Buddhas can manifest, take form as various beings, even insects, animals, and inanimate objects too such as rivers, bridges etc.

So, they say that it’s possible to open out to that and to see the world around you as full of messages of enlightenment. And - one way they put it in the Mahayana traditions is that we can only see unenlightened beings. They sometimes say that there are many more Buddhas than there are unenlightened beings. It’s like we are the few remaining unenlightened beings - still vast in number, but few compared to the inconceivably vast numbers of Buddhas. We are the few that haven’t “got it” yet. That can be quite useful if one feels pride as a result of following the Buddhist path, to reflect - that far from being remarkable as a being to have found such a path, that one may be one of the very few (by percentage) who haven’t found it yet, even though our world system is (apparently) full of such beings.