The Alt Opt Solutions box on the Linear Solver tab:

altoptsolutions

allows you to set the number of alternate optimal solutions that the linear solver should attempt to find for a linear program.  A solution is considered an optimal alternate solution if it has the same objective value as another globally optimal solution, but has different variable values.  One might think such a situation would be uncommon, however, in many instances alternate optimal solutions do exist. In fact, it is not uncommon for a large number of alternate solutions to exist for some models.

The default setting for the Alt Opt Solutions is 1, in other words, find a single optimal solution without searching for additional alternate optimal solutions.

Note: The Alt Opt Solutions is available only for linear programs.  If your model is a mixed integer linear program (MILP), then you may wish to refer to the K-Best feature for determining a selection of best solutions for MILP models.
Note: The Alt Opt Solutions feature is not exhaustive in that for numerically difficult model it's not guaranteed to find all alternate solutions, potentially finding a subset of them instead.