Compare $C(1)$ and $C(0)$. How much more do you pay when you go from 0 pages to 1 page? That increase is the cost of one page.

In a linear function like $C(p)=a+bp$, the coefficient $b$ is the rate per unit (here, per page). Identify that coefficient in $C(p)=18+0.12p$.

Type C(p) = 18 + 0.12p into Desmos, then use the table feature (tap the gear icon and select "Table"). Look at the values of $C(p)$ when $p$ increases from 0 to 1; the amount that the total cost increases by is the cost of one page.

The function is $C(p)=18+0.12p$, where $C(p)$ is the total cost in dollars and $p$ is the number of pages. This has the form $C(p)=\text{fixed fee}+\text{(cost per page)}\times p$.

The term 18 does not depend on $p$, so it is the fixed fee you pay even if you print 0 pages. The term $0.12p$ changes when $p$ changes, so it represents the part of the cost that depends on how many pages you print.

The cost for each page is the amount added to the total cost for every increase of 1 in $p$, which is exactly the coefficient of $p$ in the expression $0.12p$. Therefore, the cost of printing each individual page is $0.12$ dollars.