Trace-to-plane capacitance?
Calculating the capacitance between two tracks edge to edge is not so easy, because it's not the parallel plate scenario so it's all fringing fields. This calculator is for tracks over planes, with fringing taken into account. The same site has an inductance calculator which you should also take into account. Obviously both will depend on the length of track.

In terms of how much is too much, it will be clear when your pulses start to show a rounding effect which alter the received width of the pulses. So it depends on the tolerance of the receiver, at what point does it fail to receieve the signal correctly? You end up with a budget: the receiver tolerance has to allow for transmitter tolerance plus the transmission distortions.

In the end, you probably have to prototype it to find out, because it's not just the PCB track-ground capacitance that could affect the signal, there is capacitance between IC pins.