As a UK passport holder, an affidavit or pension letter (Call it whatever you like) from the Embassy isn't enough for immigration. You also need documentation from your pension provider stating the dates and pension figures each month.
I would guess it is for each person applying for a visa. Immigration in Thailand, accept 40,000 baht as a family. I've no idea about consulates abroad though.
I can't remember there requirements off hand. I believe they want onward flights out of Thailand, 20,000 baht per person in a bank account and booked accomodation. Somebody else will confirm right or wrong.