It's going to depend on your riser config. Looking at the manuals for the R740 and R640, it looks like you could have as many as 7 cards in the R740, and up to 3 in the R640.Subtract one if you're 1GbE card is going to be a PCI card, if it's an NDC, then these numbers wouldn't change.
  Hi, How many 25GbE ports can PER640 and PER740 servers handle simultaneously? Please note these would be in addition of a quad-port 1GbE NIC for management. Many thanks.
Maximum amount of 25GbE ports per PowerEdge Servers