Thirty eight states must need to agree to amend the articles of confederation. This is clearly stated in Article V of the Constitution of the United States where two thirds of both houses shall deem the amendment if it is necessary and when a convention is made three fourths of the legislature shall ratify the amendments laid on the table.
There are a total of 50 states in the US you multiply that with ¾ of the said states in the constitution you’ll get 37.5 votes (i.e. 38 states required).