Subject to the maximum number of seats, approved by the FAA and other regulatory bodies, airlines can cram as many or as few seats as they deem appropriate.
The original B747-100 had about 435 seats while other variants (such as the Japanese Short Range) had over 550. Same basic fuselage size, different markets.
Thompson has a
website on what they are proposing. I'm not sure if 'Economy Club' is economy or something better but they say: "Travel in Economy Club and you’ll get a 33” or 34” seat pitch – that’s more legroom in economy than BA, Virgin Atlantic, bmi and Monarch."