Martin is far ahead of me in most respects. I only have six from one obverse die. I believe there is a die with at least a dozen reverses but do not own them all. Sestertii are much less common and often worn and faulty. Finding great ones will take some looking and cash.
That answers my question, thank you. I have just bought 3 sestertius of SS and have not seen many before.
Guideline: If you see a sestertius of Septimius with complete legends on both sides and no really offensive faults, buy it. If you see one with those characteristics and a full, well struck laurel wreath, pay good money. Many of his sestertii are flatly struck and left the mint with Fine details at best. His sestertii are a bit more common from the early years of the reign. Dupondii and asses are not more common with several quite rare. Beware, however, of asses that are thinner and lighter than should be the case. These are termed 'Cast in Gaul' in the literature and should sell for a fraction of the regular coins. To my observation, many copy rare types so you really need to watch for them when buying a good type.
and if you are a big fan of SS for just £650 you can have a marble (reconstituted) bust on pedestal of him....
I bought a similar sculpture of Antoninus Pius from "sculpture_collection" on eBay. Close observers will notice that his bust is right behind my old copy of "Historia Numorum", published in 1911, which belonged to Dr. Judd (the author of the defacto reference on US Pattern coinage who was also a significant ancient coin collector)