Using the dataset abdata.dta I ran two commands, one with the noleveleq option and one without. The sample size changes from 751 with the noleveleq option to 891 without it.
Code:
webuse abdata
.
xtabond2 n L.n L(0/1).(w k) yr1978-yr1984, robust small gmmstyle(L.n w k) ivstyle(yr1978-yr1984, equation(level)) noleveleq h(2)

Group variable: id                              Number of obs      =       751
Time variable : year                            Number of groups   =       140
Number of instruments = 98                      Obs per group: min =         5
F(12, 140)    =     70.71                                      avg =      5.36
Prob > F      =     0.000                                      max =         7
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 xtabond2 n L.n L(0/1).(w k) yr1978-yr1984, robust small gmmstyle(L.n w k) ivstyle(yr1978-yr1984, equation(level)) h(2)
Group variable: id                              Number of obs      =       891
Time variable : year                            Number of groups   =       140
Number of instruments = 129                     Obs per group: min =         6
F(12, 139)    =    374.43                                      avg =      6.36
Prob > F      =     0.000                                      max =         8