A 2U server chassis can fit perhaps 2x PCI cards (with riser), vs 20x 2.5" hot-swap nvme.
Two PCIe add-in cards is normal for a 1U server. 2U servers can accommodate quite a bit more; mine has room for 6 full-height and two half-height, for a total of 8. And for 2.5" drives, a full front-loading 2U is usually 24 drives.