You are right that not all lumps have to be removed. Sebacious cysts, fluid filled cysts, lipomas (fatty tumours), warts etc do not necessarily have to be removed unless they grow very big and /or interfere with circulation or muscle movement etc.
But it's often difficult to tell what a lump is unless it's analyzed. Sometimes -but not always -the only way to know for sure what it is, is to do an incisional biopsy (remove it)
There is another option, though not 100% reliable...and that is a fine needle aspiration , or needle biopsy. This is hardly invasive at all, and just takes a few cells from the lump with a fine needle. It needs no anaesthetic, sedative or special preparation for the dog.
It can differentiate between fluid, sebacious cyst, possible cancer, or fatty tissue. But as I said it is not 100% reliable, and a sample should be taken from more than one place on the lump.
So basically you can't know what all those new lumps are....unless the vet has told you what he/she thinks they are.
If you've been told that they more than likely are benign (flid filled cysts for example) then no, they don't need to be removed. If that's the case, just keep an eye on them, and if they get any bigger, get them needle-biopsied.
But do beware because mast cell tumours can look like pretty harmless big warty spots! If you are unsure about any of the lumps, take her back to the vet to get them checked.
Yes you can leave them if the vet is sure they are benign.