Who says "v'Li Mah Yakru Re'echa"?
Rashi: Keneses Yisrael says, how honored in my eyes are the Tzadikim in every generation.
Radak: "Yakru" means that I cannot attain it, like "u'Devar Hashem Hayah Yakar ba'Yamim ha'Hem" (Shmuel I, 3:1). "Re'echa" is Your thoughts, i.e. Your intent in creation.
Malbim: Philosophers said, since man has choice, Hashem's knowledge of the details of man's actions must increase and change. This contradicts His unity, for He is His knowledge! Moreh ha'Nevuchim brings such questions. The answer is, Hashem knows all this from the beginning, for He is the source of all that occurs. Man, who is tied to physicality, cannot fathom this - "v'Li Mah."
What is the meaning of "Mah Atzmu Rosheihem"?
Rashi: [How great is] their number. This is like "Ki Sisa Es Rosh Bnei Yisrael" (Shemos 30:12).
Radak: They grew in number. The next verse says "Esperem". Atzam can refer to strengthening or increase - "Atzmu Li Almenosav me'Chol Yamim" (Yirmeyah 15:8). "Rosheihem" are their Klalim, like "Ki Sisa Es Rosh Bnei Yisrael" of Your intent in the creation are many, and all the more so the details!
Malbim: Your thoughts (knowledge of actions - refer to 139:17:1:3) are very dear to me - they are too great for me to understand. If I would count them, they would exceed the sand of the sea (18)! How can there be knowledge of something without end? Than I realized that Rosheihem (their source, of everything that will transpire) is in Shamayim, in absolute unity.
