Constraint on count of variables >0 in JuMP?

Specific Domains Optimization (Mathematical)
2

Here you go:

T, u_max, u_non_zero_max = 3, 4.0, 2
model = Model()
@variable(model, 0 <= u[1:(T+1)] <= u_max)
@variable(model, z[1:(T+1)], Bin)
@constraint(model, [t in 1:T+1], u[t] <= u_max * z[t])
@constraint(model, sum(z) <= u_non_zero_max)