“Any Tehillim or only for a complete Sefer?”
The beautiful, shortened Yehi Ratzon for Shabbos and YO”T is found at the end of most sifrei Tehillim and is said after completing the corresponding sefer/sfarim.
The 150 kapitlach are divided into 5 sfarim:
Sefer Rishon- 1-41 kineged Braishis
Sefer Shaynee- 42-72 kineged Shmos
Sefer Shlishi- 73-89 (a short one!) kineged Vayikra
Sefer Revii- 90-106 (also short!) kineged Bamidbar
Sefer Chamishi-107-150 kineged Dvarim