The issue has come up again recently: why are there often two different numbers for the same psalm? The Hebrew Bible counted the psalms one way. The Septuagint translation of the Hebrew Bible into Greek counted many psalms slightly differently. The early Christians used the Septuagint, and then went on to translate into Latin (the Vulgate) following the Septuagint numbering. Protestants and Anglicans (Episcopalians) tend to use the Hebrew numbering. Catholics and Orthodox tend to use the Septuagint numbering.
Psalms 1-8 are the same in Hebrew and the Septuagint.
Psalms 9 and 10 in Hebrew are combined as Psalm 9 in the Septuagint.
Psalms 11-113 in Hebrew are Psalms 10-112 in the Septuagint (Hebrew-1 = Greek).
Psalms 114 and 115 in Hebrew are combined as Psalm 113 in the Septuagint.
Psalm 116 in Hebrew is divided into Psalms 114 and 115 in the Septuagint.
Psalms 117-146 in Hebrew are Psalms 116-145 in the Septuagint (Hebrew-1 = Greek).
Psalm 147 in Hebrew is divided into Psalms 146 and 147 in the Septuagint.
Psalms 148-150 are the same in Hebrew and the Septuagint.
If you have a mnemonic for this, do let us know!