Blackstone Group LP, the private equity giant with ambitions for the largest-ever infrastructure fund, is seeking as much as $10 billion to supplement a $20 billion anchor commitment for the pool before it starts investing, people with knowledge of the plans said. The firm is planning to start with $5 billion of outside capital and as much as $5 billion from clients with separately managed accounts, said the people, who asked not to be identified because the process is private.