How to block part of a method to be processed only once per request with . net core

Question

i have to increment a column in the database and this increment may depend on the situation, for example increment according to the user, I am using Jmeter to send 100 requests at the same time, but all of them are running at once, at the time of saving I needed to make a logic to increment the value of the column I tried to use lock and Semaphoreslim, but when sending the requests the server is stopped and does not execute anything when it arrives in lock or _Semaphore. Wait() requests do not pass, I am using Nhibernate, how can I make this type of function block part of a method to be executed once each.

private static readonly object _lock = new object();

public override DepositoProduto Save(DepositoProduto entity)
{
    lock (_lock)
    {
        using (var transaction = Session.BeginTransaction())
        {
            //aqui a função do auto incremento pode depende no caso coloquei um exemplo simples
            entity.Codigo = All().OrderByDescending(x = > x.Codigo).Select(x = > x.Codigo).First() + 1;

            //aqui realiza o save no banco
            transaction.Commit();
        }
    }
    return entity;
}

Using Semaphoreslim

private static SemaphoreSlim _semaphore = new SemaphoreSlim(1);

public override DepositoProduto Save(DepositoProduto entity)
{
    _semaphore.Wait();
    using (var transaction = Session.BeginTransaction())
    {
        //aqui a função do auto incremento pode depende no caso coloquei um exemplo simples
        entity.Codigo = All().OrderByDescending(x = > x.Codigo).Select(x = > x.Codigo).First() + 1;

        //aqui realiza o save no banco
        transaction.Commit();
    }
    _semaphore.Release();
    return entity;
}