To list multiple transactions in a single letter, do the following:
On step 2 of the letter creation process, set the Create One Letter Per option to either Constituent or Household. Setting it to Transaction will create a separate letter for each transaction.
You can also embed a data table into the letter to display multiple transactions. To do this, click inside the body of the letter where you would like to add the table, then click on Insert Bloomerang Data Table. This will embed a table into your letter that will list all of the transactions for the constituent or household that you filter for.