As many as they need there are no limits According to Football League rules section 52 Temporary Loan Transfers. Rule 52.3.2 states. Up to 4 loan players(Standard, Emergency or Youth), may join one Club from another Club in any Season. An additional Youth Loan player may be added to this figure.
35 American players are in the English premier league.
i just know Claudio Pizzaro loan from Roma to M.City season 2011-2012