How much faith do you need to become a buddhist? Faith in hell and heaven, Karma, rebirth, other realms etc. because all of them can't be proved via scientific method. If you say that you can see them by yourself, how do you know that it is not an illusion like when we have a lucid dream. Thanks
In hardcore ('supramundane') Buddhism, it is taught the cause for the generation of faith is your personal experience of suffering or dissatisfaction with ordinary worldly life. To quote:

Faith, monks, also has a supporting condition, I say, it does not lack a supporting condition. And what is the supporting condition for
faith? 'Suffering' should be the reply.
Upanisa Sutta

~

And what is the noble search? There is the case where a person, himself being subject to birth, seeing the drawbacks of birth, seeks
the unborn, unexcelled rest from the yoke: Nibbana. Himself being
subject to aging... illness... death... sorrow... defilement, seeing
the drawbacks of aging... illness... death... sorrow... defilement,
seeks the aging-less, illness-less, deathless, sorrow-less, undefiled,
unexcelled rest from the yoke: Nibbana. This is the noble search.
Ariyapariyesana Sutta

In addition, the scriptures explain hearing the True Teaching is a cause of faith.

Faith, too, has its nutriment; it is not without a nutriment. And what is the nutriment of faith? 'Listening to the True Teaching,'
should be the answer.
AN 10.61

The hardcore scriptures explain individual people are drawn to eachother due to the individual genetic (dhatu) composition of their bio-psychology (nama-rupa). For certain individuals, hearing about unknowable things in Buddhism will turn them away from Buddhism therefore the purpose of Buddhism will never be achieved by anyone on the planet Earth (since craving for future lives, similar to how many Christians are interested in living forever, is contrary to the path to Nibbana).
Therefore, on the hardcore level, faith in external hells and heavens, karmic-rebirth, other realms etc, is not required, which is why in the 1st three sermons attributed to the Buddha, which gave rise to many enlightened beings, these things were not mentioned at all.
If you say that you can see them by yourself, yes, in hardcore Buddhism, this is considered to be an illusion like when we have a lucid dream. To quote:

Now suppose that a man desiring heartwood, in quest of heartwood, seeking heartwood, were to go into a forest carrying a sharp ax. There
he would see a large banana tree: straight, young, of enormous height.
He would cut it at the root and, having cut it at the root, would chop
off the top. Having chopped off the top, he would peel away the outer
skin. Peeling away the outer skin, he wouldn't even find sapwood, to
say nothing of heartwood. Then a man with good eyesight would see it,
observe it, &amp; appropriately examine it. To him — seeing it, observing
it, &amp; appropriately examining it — it would appear empty, void,
without substance: for what substance would there be in a banana tree?
In the same way, a monk sees, observes, &amp; appropriately examines any
mental fabrications that are past, future, or present; internal or external;
blatant or subtle; common or sublime; far or near. To him — seeing
them, observing them, &amp; appropriately examining them — they would
appear empty, void, without substance: for what substance would there
be in fabrications?
&quot;Now suppose that a magician or magician's
apprentice were to display a magic trick at a major intersection, and
a man with good eyesight were to see it, observe it, &amp; appropriately
examine it. To him — seeing it, observing it, &amp; appropriately
examining it — it would appear empty, void, without substance: for
what substance would there be in a magic trick? In the same way, a
monk sees, observes, &amp; appropriately examines any consciousness that
is past, future, or present; internal or external; blatant or subtle;
common or sublime; far or near. To him — seeing it, observing it, &amp;
appropriately examining it — it would appear empty, void, without
substance: for what substance would there be in consciousness?
Phena Sutta

In conclusion, if the mind can see clearly the illusionary nature of all mental phenomena &amp; sense objects of consciousness, it should be already enlightened (therefore does not require to ask questions about faith in Buddhism).