Right now, it costs around $3 to send Bitcoins. But as the number of users increase, this can also go up. If you remember, during the peak of the bitcoin bull run in 2017, we had fee ranging from $30 to $50 for a single transaction. The mining pools were making a killing back then. But the strategy backfired, as the users shifted to ETH, LTC and BCH.
number of users does not affect the transaction fees but its the demand or the increase of the price , thats what trully affect the transaction fees . also , not all wallet/exchange have the same fees . some have less inexpensive fee because they use low priority or slower speed processing while others dont like waiting so they use high fees
If the number of users increase, then in all probability the number of transactions can also go up. And then, it depends on the exchange rate was well. The transaction fee may remain the same in terms of BTC. But its value in USD can go up if the prices rise. And I always use wallets where I can set the fee myself. If I think that the default fee is too high, I manually set it to a lower amount.