Hi Esmé! Welcome to the Aerocene community! Thanks for your question! The main point to know about building a Museo Aerosolar is that it all begins with an array of plastic bags with a 3:1 aspect ratio. For example, check this Museo we built in Buenos Aires a year ago: Aerocene - TECAT - #3 by Joaquin
We pasted bags to form a 12 x 4 mts patch, total balloon area 48 square meters. The balloon size once inflated was roughly 3mts tall, 3mts wide. Then how many bags depends on the average size of the bags you are going to use, which could be quite varying.
Then if you take a typical plastic bag that measures 30x40cm, unfolded 60x40cm > 2400sq centimeters, or 0,24 sq meters. So 48 sq meters divided by 0,24 sq meters > you would need 200 of these bags to build this balloon! Hope this helps 
It would be amazing if you can do a short self introduction here > Introduce yourself! We'd love to meet you :) - #32 by camilla