I was a single parent when dd1 was young. It was fine when I had my family's support, so my dad did childcare, I could ask my parents for help with things, etc. Then I moved for a job, and that year I moved away and was a single parent with no support, DD was disadvantaged then. Then we moved in with my now DH, and suddenly I didn't feel she was disadvantaged again.
So I wouldn't say it's about the number of adults living in the house, it's about the number of adults involved in looking after the child who care for/love them.
The financial side makes a HUGE difference. The year I was on my own with no support, I was paying rent, bills, childcare all on one wage and it was incredibly stressful. I was just over the threshold for any help. When I lived near my parents they did my childcare so I didn't have that bill. When we moved in with DH the rent etc were shared so the finances were much easier.
There are a lot of factors of living in a single parent household that are not a reflection on the single parent, but are a reality of life and a signal that the child (ren) could be disadvantaged. Poverty, one person trying to do all the work and household stuff, leaves less time for adult-child interaction.
It's not a reflection on the single parent, most often they are doing an amazing job in very difficult circumstances. The disadvantage comes from the difficult circumstances that come from the one adult household, like the lack of money and time, or the difficult co parenting relationship, or having to swap between 2 houses for contact and the difficulties in being organised and in a routine due to that.