which drive fit?

Usually a 5i20 is all you need unless you have a ton of I/O then the 5i22 with the 96 I/O is used... but this all depends on what kind of drives you have. Do they take a velocity input like +-10v?

John

5i23 can be seen as a somewhat "souped up" 5i20, is has a bigger FPGA so can run some more complicated firmwares. It might be worth the $30 additional price. However the 5i22 is a _lot_ ore expensive and unless you really need the extra IO is seems overkill for EMC2. For EMC2 use it would probably make sense to use two 5i20s than one 5i22 because you could load two different firmwares so would have more flexibility as well as 6 rather than 4 headers.
As far as I know all the EMC2-supported firmwares fit in the 200k chip on the 5i20.

You can also look at the 7i43, that is much cheaper, and can be easier to cable as it connects to the controller via the Parallel port. It can have a 400k FPGA and has 48 IO pins which is plenty for most machine tools.

If you have brushed servos then bear in mind that you can connect two 7i29 cards to a single FPGA board header, so 1 x 7i43 + 2 x 7i29 + 7i37TA is a perfectly reasonable and cost-effective setup which can connect directly to the motors.

Depending on the size and voltage of your motors, the 7i29 might be overkill.

What motors do you have? Do you already have drives?

It is possible to avoid the conversion to +/-10V signals. Mesa have a couple of different brushless motor drives.
(Unless you are planning to use cheaper, ex-industrial drives from eBay or similar).
The 8i20 communicates via a serial protocol to the Mesa IO card, and the 7i39 works with individual PWM signals on a 50pin ribbon header cable.
Both will be supported in the pending 2.5 release of EMC2 (which is available for use and testing) but are not currently supported by the 2.4.6 version, it appears.

Otherwise you probably want to use a 7i33 to create the analogue voltages and condition the encoder