How big current I can commutate using D-sub connectors?

The obvious answer is to READ THE DATASHEET. The current rating is one of the important specs for any electrical connector, so even marginally reasonable datasheets will tell you this.

I haven't looked up the datasheet for a "D-sub" connector since that's your job (you also need to explain which D-sub connector), but anything normally understood as a typical "D-sub" connector, like a DB-9, isn't going to do 10 A per pin. Not even close.

You can parallel multiple pins of a connector to get effectively higher current rating at the expense of fewer connections. You should derate the total current capability somewhat since the pins won't share the current equally, but you don't have to go too far with that since individual connections are mostly resistive.

Have a datasheet: http://portal.fciconnect.com/Comergent//fci/drawing/c-dsub-0071.pdf for http://www.digikey.com/product-detail/en/865609SLTLF/609-1467-ND/1001781 which claims a max of 5A per contact!

Personally your 45A total seems like a horrifyingly high number, but if you wanted to split your 10A across 10 power and 10 ground connections in a 25 way connector that seems OK.

Don't forget the wire gague; http://www.powerstream.com/Wire_Size.htm says 20 AWG or larger (smaller AWG number), but it also needs to fit the D connector.

On Megasquirt EFI they regularly run currents of 5A through a single or pair of pins on a D-37 connector. I've run the system on several cars for years and it's been very reliable.

As someone said above, proper manufacturers like Tyco, Amp, ITT Canon adhere to the spec & will publish a data sheet you can believe in.

However, they also do mixed-signal D connectors which have different contacts allowing for high-current, co-axial cable, and normal signal pins in one connector body. They cost more but may be a better solution.