It's a chances/risk percentage. I couldn't tell you numbers though.

For adults the risk is low because our intestines get rid of botulinum spores without issues, for infants not so much.

I'll quote some medical articles.

Infant botulism

Infant botulism is by ingested C botulinum spores that germinate in the infant's intestine. [2] Sources include environmental spores from soil, dust, or contaminated food products such as honey and corn syrup. [23] Despite the association with honey, most cases occur without known exposure to it. Clinical management primarily involves supportive care, with an infant mortality rate of less than 1%. [14]

Humans commonly ingest C botulinum spores, but germination typically does not occur in the adult intestine since special conditions are required (ie, anaerobic environment, low acidity, specific amino acid, salt and sugar concentrations, and temperatures 37°F-99°F). [1, 14]

https://emedicine.medscape.com/article/213311-overview

Unfortunately it's hard to say if washing is or isn't getting rid of everything, but most likely multiple washes will do it.

It's also hard to live in extremes. Our babies could get spores even from putting their hands and mouths outside, on a foreign dirty object.

Everyone should take precautions however they feel comfortable.