Question
How difficult or rare is it to get human life?
Answer
The scriptures definitely say that human life is rare or difficult to get. But IMO it will be hard to answer the "how much" part in your question.
Given below are a few relevant verses, which are the words of Lord Shiva.
Atra janma sahasreshu sahasrairapi PArvati | KadAchit labhate jantur mAnushyam puntyasanchayAt ||
Meaning
[Lord Shiva says to Goddess PArvati] Thousands and thousands of jivas are getting equally numerous births, but among them only rarely (kadAchit) one gets a human birth due to its accumulated merits (punyasanchaya).
SopAnabhutam mokshashya mAnushya, prApya durlabham | YastArayati nAtmAnam tasmat pApatarohatra kah ||
Meaning
Human life, which is like a staircase to liberation (moksha), is rare (durlabham) to achieve. Who is more sinner than the one who gets it but fails to utilize it [to the fullest].
KulArnava Tantram, UllAsah 1, Verses 15,16.
Another relevant verse is the following:
Chaturvida sarirAni dhritvA sahasrashah | SukritAnmAnavo bhutvA jyAni chenmokshamApluyAt ||
Meaning
The jiva upon obtaining the four kinds of bodies, repeatedly and for numerous times finally obtains the life of a human, and if in that life he attains knowledge [of the Self or atma jyAna], he attains liberation (moksha).
KulArnava Tantram 1-14
Commentary on the chaturvidhasarirAni is as follows:
"JarAyujAndajAtAni SvedajAnyudvidAni cha"- Plants, Svedaja (born from Sveda?), Andaja (born of eggs) and JarAyuja (born of a Yoni) are the four kinds of bodies.
The KulArmava Tantram and several other Agamas also talk about the 84 lakhs of bodies viz "chaturshitilaksheshu sarireshu" but even with that info we can't seem to answer the "how much" part.
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