Answering your questions:
There are only 2 PCIe slots on this board allowing you to use only 2 riser cards. The riser cards are pass through only to the PCIe slot on the board.
The mezzanine port (I/O module carrier) supports both a SATA M.2 storage device and a 8x PCIe 3.0 devices. You can check the Technical Product Specification document on page 18
The S2600TP family board does support VMware ESXi 6.5 at a basic installation and compatibility level. You can check this info on this URL
Hope this helps
I would like to use riser slots 1 & 2 for standard PCIe expansion slots what are the part numbers for the riser cards? I couldn't find them in the manual.
The PCIe riser part number is:
PCI express riser FHW1U16RISER2.
You could get the spare parts number list from the following document: https://www.intel.com/content/dam/support/us/en/documents/server-products/server-boards/S2600TPR_HNS2600TPR_H2000G_Configuration_Guide_v2_1.pdf
Thats all the questions I have for now. Will post more if I have further questions. Thanks for your help.