RSTP and MSTP are bound together.
To your question:
Yes and no.
On 3550 you will have one spanning tree. However if you will have classic STP switch(device) connected to port of 3550, 3550 will comunicate with that switch over classic PVST protocol. so basicaly it will proces as many BPDUs as many VLANs you will have...
If you connect MSTP capable switch, they will communicate depend if they are in the same region, resp. instance mapping via as many BPDUs as many different instances they have....
hope it'll help.