DALLAS — Blockbuster is planning to close as many as 960 stores by the end of next year. That would shrink the video rental chain by more than 20 percent as it struggles against stiff competition from Netflix and Redbox. The store closures disclosed in documents filed Tuesday shows Blockbuster is having to cut much deeper than it anticipated to save money and keep its lenders happy. Blockbuster now expects to close between 810 and 960 of its U.S. stores through 2010, up from the 380 to 425 stores that normally would be closed. If Blockbuster hits the high end of the new target for store closures, it will represent 22 percent of its 4,356 U.S. stores. The Dallas-based company has closed 276 stores so far this year, including one on Spokane’s South Hill. Blockbuster now has five locations in Spokane and one in Spokane Valley. It is unknown whether any area stores will be closed after this latest announcement.