all devices on a sky130 project must map to existing silicon devices. For capacitors you must decide if you want to use MOS capacitors or MIM capacitors. MIM (Metal Insulator metal) are usually the best choice. You must set W and L and xschem shows the approximated calculated capacitance.
for parallel you can set the MF parameter (multiplicity factor) or use vector instance name, like C1[9:0], representing 10 capacitor instances C1[9], C1[8], ..., C1[0]. For series connection there is no attomatic attribute, just draw the number of series caps.