You have a fixed number (N) of flags *per day*. Thus, when you use one the counter will always decrease and will reset the next day (indpendent of the quality of your flags).
However, the number N does depend on your flagging history and your number of points.

Note that for most actions on MathOverflow (and SE sites) there is a rate limit; see The Complete Rate-Limiting Guide for all the details and On the recent changes to flagging and limits for the number of flags specfically. In brief, it is between 10 and 100, you start at 10 and each 2000 points and each 10 helpful flags give you an extra flag per day.

For seeing what happened to your flags you can click on the number after "Helpful flags" (which is shown if it is nonzero) on your user page, or go to https://mathoverflow.net/users/flag-summary/xyz where xyz should be replaced by your usernumber.

Note that having a "disputed" flag does not necessarily mean that a moderator thought you were wrong; roughly, if there are contradictory flags all become "disputed."
And, there are some other details where the feedback could be confusing for technical reasons. For further deatils on flags see this meta.math.SE post https://math.meta.stackexchange.com/questions/4328/capture-the-flag-faq-on-flagging (which should more or less apply to MO).

onlyfree on questions; on answers they still cost1point (yet subtract 2 for OP); earlier this cost of 1 point applied to both types of posts. $\endgroup$ – user9072 Mar 30 '14 at 15:45