Sivanandi welcome to ASB! To answer your question, you do not have to start with just one Pokemon. Instead, as detailed
here, you get 6 Currency Counters, or CC, to spend to purchase the Pokemon you want at the start. The only Pokemon purchasable are first-stage Pokemon or Pokemon that do not evolve (excluding Legendaries). Their CC costs are detailed
in this spreadsheet, which also contains their stats as well as descriptions for all moves, abilities, and items in ASB, as well as the movepools of the Pokemon made by the CAP Project, which can also be used here.
I hope you decide to stay with ASB! :D