Sample used in calculating the sample regression function
It is not very clear what you're asking, but if the model is of the form

$$
y_i = \beta_0 + \beta_1 x_i + \epsilon_i \tag{1}
$$

Then the sample regression function (SRF)

$$
\hat{y}_i = \beta_0 + \beta_1 x_i \tag{2}
$$

indeed only considers one value of $x_i$ at the time. If you have a situation in which several observations $y$ are associated with the same value $x_i$, then a linear model is perhaps not appropriate for describing your data