I think I had about 25 in total, but that's a total guess. I do know I bought around half a dozen extras compared to what I had with just one DC.
The thing is, if you have two in nappies, you fill a load of washing quickly. So as long as you are organised and have a type that dries quickly (we had bumgenius, which obviously dry fast because they come apart) you don't need many extra. If you previously did a wash every 2-3 days, you'll do one every 1-2.
It's lots of wash loads, but was only a few months. I never found a newborn needed 10 nappies a day though. And I did use disposables at night during this phase.
I'm currently expecting DC2 (similar age difference, DS1 won't be potty trained yet) and have got about 35-40 nappies in total now to serve both DC in nappies. We started with DS1 when he was 11m and had about 16 nappies, which was fine for washing every other day (and now at 22m he only uses about 4 a day, we use disposables at night). But a newborn potentially needs up to 10 nappy changes a day so only 20 nappies for 2 DC seems awfully short, unless you wash every day and they're quick drying in which case it might work?
All DS1s nappies were bought new, but I bought the additions to my stash second hand from eBay and a few Facebook nappy groups. Some real bargains to be had.
We use birth to potty washable nappies with DC1, who will be 21mo when DC2 arrives, so still in nappies unless miracle occurs. All advice on this type of nappy is you need 20 for your child, and we found this was true when DC1 was newborn, but not anymore. Has anyone on here had any experience of using washables with two DCs? How many did you need when second DC was born? I really don't want to have to shell out for another 20 if I don't need to, but can't risk being caught short. All help much appreciated!