This may be a very rookie question. Say I have a network card with bandwidth limit 100MB/s, so is it possible that in/out bandwidth reach that limit at the same time? Or I'll have this inequality at any point of time: in bandwidth + out bandwidth <= 100MB/s
First, your network card is probably 100Mb/sec not 100MB/sec. Ethernet is the most common wired network type by far, and it commonly comes in 10, 100, 1000 mega bits per second. A 100 megaBIT/sec ethernet interface is roughly capable of 12.5 MegaBYTES per second.
If you're plugged into an ethernet switch, you're most likely going to be connecting in Full Duplex mode. This allows both ends to speak to each other simultaneously without affecting the performance of each other.
You'll never quite reach the full advertised speed though, a Gigabit network interface (1000Mb/sec) will usually be able to transfer in the high 900's each direction without problem. There are a few things that cause overhead that prevent you from reaching the full speed. Also, many lower end network cards or computers struggle to reach the full speed, so you won't always be able to reach this.
If you're plugged into an ethernet hub, only one end can be talking at a time. There, in + out can't go higher than the link speed, and is typically far lower because of collisions. It's really unlikely you can find a hub anymore unless you're really trying to, switches are pretty much the only thing you can buy now outside of exotic applications.
TL;DR: You're almost always using full duplex mode, which allows up to (but usually less than) the advertised link speed in both directions simultaneously.