Normally speaking, a tent used for gathering with chairs without any table, each people will take about 1㎡.
Eg: 100 people hold a meeting with 100 chairs will need 100㎡ and 20㎡ lecture desk area, you will need 120㎡ totally, so one set of 8x15m tent is OK.
A tent used for gathering with chairs and tables, each people will take 1.5㎡.
Eg: 100 people hold a meeting with chair, table, and 30㎡ lecture desk area, you will need 180㎡ totally, so one set of 10x18m tent is OK.
The above calculation method is just for reference. Different outdoor events usually have different conditions. Pls tell us your requirements, we will provide professional help for you.