The last issue (#23) is tough to find - great shark cover! Oddly the Alan Class web-site only lists 15 issues. And Denis Gifford's Price Guide only lists to #21.
Some issues were also published without cover numbers which causes much confusion! These issues are: #1-4, #6, #10, #12 and #13 to my knowledge. I suspect #5 as well. What proportion of un-numbered to numbered copies I don't know.
As with all Alan Class titles, there are no dates on the cover or inside so this is very frustrating. Weird Planets is advertised on the back of Uncanny Tales #21 along with Out of This World ie. in 1965.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.