No. You do not need a letter for every out-of-pocket expense. You will need a letter for each individual donation of $250 or more.
If all your individual donations are less than $250 each, but together are more than $250, you only need one letter.
The IRS says:
You can claim a deduction for a contribution of $250 or more only if you have a contemporaneous written acknowledgment of your contribution from the qualified organization.
If you made more than one contribution of $250 or more, you must have either a separate acknowledgment for each or one acknowledgment that lists each contribution and the date of each contribution and shows your total contributions.
In figuring whether your contribution is $250 or more, don't combine separate contributions. For example, if you gave your church $25 each week, your weekly payments don't have to be combined. Each payment is a separate contribution.
See Contributions of $250 or More
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