Question: I am trying to find out the importance of the number 20 in the Hebrew language or customs. Can you help?
Response: Well, twenty is not generally considered to have an important "symbolic" meaning (cf. Biblical Numerology by J.J. Davis: Baker, Grand Rapids 1968). In the Hebrew system of numerology, 20 is represented by the letter kaph (roughly our k or ch). It is also true from usage that 20 is generally a number of completion and sufficiency: twenty is age at which a man was eligible for military service (Num.1:3ff.), and is once mentioned as an age at which a Levite was considered old enough for service (Lev.27:3ff.; although 30 was usually the more important milestone), and 20 is also a number that figures heavily in the tabernacle (once again, a full and complete "set" so to speak: Exodus chapters 26-27). There are other numbers in Hebrew which do this same sort of thing (40 comes to mind), but based upon the use of twenty as adequate age for service and adequate number for serviceability (e.g., sockets and pillars in the tabernacle) one might argue that this is the main symbolic point (if any) that is generally found in the Old Testament, where we are to derive such meaning.
Hope this is of some help - in addition to the bibliography above, you might also try a good multi-volume Bible encyclopedia (Interpreter's Dictionary has a long article on biblical numerology, and the Anchor Bible Dictionary is usually good on such things as well).
You might also have a look at the following links:
Number 13, Letters, and the Mark of the Beast.
The Number of the Beast (666)
The Number Twelve
The Number 40
Yours in Christ,