I was said that it is Haraam to say: We will have just two (three, four, five, etc.) children Inshaa' Allaah. Is it? We thought, that saying Inshaa' Allaah means, that we -in the end- trust in Allaah decisions, but we were said it is like asking Allaah just for two (three, four) children and it is Haraam because it means we don't want to have more than two (three, four, etc.) children. I read all your Fataawa concerning pregnancy, family planning, etc., and I didn't find answer for my question. Please answer my doubt. We are searching for Allaah forgiveness, if we are wrong by using such sentence.

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not very clear to us what you meant by your question.  Nonetheless, it should be mentioned that permanent birth control after having a given number of children is forbidden, as long as there is no sound religious reason for doing so.  That is because birth control is contrary to the objectives of marriage which is to multiply the progeny and so forth.  However, there is no harm in temporary birth control if it is done for a valid reason.  Nonetheless, when a Muslim knows that something is forbidden, he must avoid it.  It appears, Allaah willing, that there is no harm in the expression which you mentioned as it is conditioned on the Will of Allaah, even if this is said in regard to the forbidden birth control (i.e. the permanent one), as it is not understood that one is determined to do so.  As regards the permissible birth control (i.e. temporary one), especially if it is done for a sound reason, it is permissible.  Besides, conditioning future events by Allaah's Will is something required.  Allaah Says (what means): {And never say of anything, "Indeed, I will do that tomorrow. Except [when adding], "If Allaah wills."}[Quran 18:23-24].

Allaah Knows best.