The number of expiations depends on the number of oaths
Question

                I am a mother and I swore many oath to make my children do things and sometimes the oath is broken. Should I offer kafaarat yameen (expiation for breaking a oath) once or what should I do?.
Praise be to Allah.
    It is makrooh to be careless in swearing aaths, because 
    Allaah says (interpretation of the meaning): 
“And (O Muhammad) obey 
    you not everyone Hallaaf Maheen (the one who swears much and is a liar or is 
    worthless)”

    [al-Qalam 68:10]

    This indicates that the one who swears oaths a great deal is 
    to be criticized, so try not to swear oaths too much, out of respect towards 
    Allaah and so as to protect your oaths. 

    You should note that swearing oaths a great deal for 
    everything, minor or major, leads to weakening the value of oaths and vows 
    in people’s eyes, and there is no guarantee that this will not lead to 
    making false oaths, and it also indicate that one does not fear Allaah 
    completely. 

    With regard to the oath that you have sworn, they may be 
    interpreted in one of two ways: 

    1 – When you swore the oath you meant to swear an oath, in 
    whole or in part, i.e., you meant to swear a binding oath. In this case you 
    have to offer kafaarat yameen. A binding oath is one which a person swears 
    concerning something in the future, that he will do something or not do 
    something. 

    2 – When you swore the oath you did not mean to swear an oath 
    as such. This comes under the heading of idle oaths. The scholars differed 
    as to the precise definition of idle oaths (laghw al-yameen). The most 
    correct view is that the idle oath includes the following: 

    1-That which the speaker utters 
    unintentionally, such as when a man says, “No, by Allaah, I will never go” 
    or “Yes, by Allaah, I will go.” This is the view of the Shaafa’is and 
    Hanbalis.

    2-When a person utters an oath 
    thinking that he is speaking the truth, then he realizes that it was not 
    true. This is the view of the Hanbalis.

    3-Shaykh al-Islam added to the 
    definition of an idle oath cases where the one who swears the oath thinks 
    that the one concerning whom it is sworn will not go against him, then he 
    does go against him. By the same token a person does not break his oath if 
    he swore that another person would do something by way of honouring him, not 
    by way of compelling him. He said: Because it is like a command; a command 
    is not binding if it is understood to be by way of honouring, because the 
    Prophet  (peace and blessings of Allaah be upon him) commanded Abu Bakr 
    to stand in the row but he did not stand. See Majmoo’at al-Rasaa’il 
    al-Fiqhiyyah by Shaykh Khaalid al-Mushayqih, p. 234.

    Based on the above, if all or some of your oaths were 
    binding, then you have to offer kafaarat yameen, but do you have to offer it 
    once or several times? That depends on the content of the oath. If all your 
    oaths had to do with the same thing, then you have to offer expiation only 
    once. But if you swore oaths concerning a number of things, such as saying, 
    “By Allaah, I will not eat today” and “By Allaah, I will not drink today” 
    and “By Allaah, I will not travel today”, then you have to offer expiation 
    for each of these things if you did it. If you ate and drank and travelled 
    then you have to offer three expiations. If you swore one oath concerning a 
    number of things, such as if you said, “By Allaah, I will not eat or drink 
    or travel,” then you have to offer expiation once for doing one or all of 
    these things. (op. cit., p. 266). 

    But if all or some of your oaths were idle oaths, then the 
    majority of scholars have stated that no expiation is required for breaking 
    an idle oath, because Allaah says (interpretation of the meaning): 
“Allaah will not call you 
    to account for that which is unintentional in your oaths”
    [al-Baqarah 2:225] 

    And Allaah knows best. 

    See also questions no.
    45676,
    34730.