Any PCIe card, including PCIe SSDs, sound cards, network cards, RAID controller cards, etc., installed on the board will affect the PCIe lane bandwidth as follows. If you have a:

Z87 FTW, Z97 FTW* :
Single card: 16x\8x\4x\1x
Two cards: 8x\8x\4x\1x
Three or four cards: 8x\4x\4x\1x

FTW boards support 2-Way SLI only on slots 1 and 2.

Z87 Classified
Single card: 16x any slot
Two cards: 8x\16x\8x\16x\8x
Three or four cards: 8x\8x\8x\8x\8x

 Z97 Classified:
Single card: 16x\16x\8x\16x\8x
2 cards: 16x\16x in slots 2 and 4
                16x\8x in slots 1 and 2
                 8x\8x in any other 2 slots
3 cards: 8x\8x\8x in any slots
4 cards: 8x\8x\8x\8x in any slots

*Z97 FTW: If you use the m.2 slot, this will count as a PCIe card, and drop the lanes down accordingly.