Are we 100% sure the engine numbering is based on when they bought instead of something else? Wouldn't that mean that Sir Topham is mostly renting engines from 1920-1950?

Just going from 1920 to 1950 when Toby showed up, that would mean Sir Topham Hatt only owned 6 engines excluding Toby with everyone else being a Loan or something else, would renting all those engines not be expensive?