Practical limits of how many LEDs can you use to replace halogen 12V?
While most 110-240V LEDs are nasty complex loads due to their AC/DC power supply, many 12V LEDs are probably no more complex than the LED and a resistor, or possibly the LED, another diode or 4, and a resistor for 12VAC service. That sort should run fine of a 12V supply at full ratings and with no need for a regular halogen bulb in circuit.

If your 12V LEDs are straightforward "bulb replacements" or devoid of a power supply circuit between the 12V supply and the LED, they will probably behave as above - after all, when the halogens you put in circuit burn out, that's where you'll be anyway. If they have a more complex power supply, the manufacturer would be the best bet to determine the correct deployment on your power supply. In all likelihood, they will be fine.