As far as I know the 'MC-36' was a competing design to the Bilbao armoured car for the Assault Guards, built by the
La Sociedad Comercial de Hierros on a commecial H-S T-69 truck chassis, but was not selected due to the cost.
Despite the heavy emphasis on Soviet, Italian and German help to the respective sides during the SCW, they did not send as many tanks as people might think and those they sent had previously been in use by their former owners. Armour of any type was in short supply and armoured cars even more so. The Republicans had small numbers of the Soviet-supplied BA-3/6 'gun' armoured car from October '36, the Nationalists had nothing comparable.
Unserviceable T-26 tanks usually had little wrong with their turrets (the Nationalists only put about half of those they captured back into use, the rest they used as spares), so there were captured tanks with good turrets on chassis that would never run again.
I assume someone had a brainstorm about re-using the turrets to replace the MG cupola on the 'MC-36' and Olé!
The actual total numbers are hard to determine. Numbers of between 5 and 15 (I'm going with the five that have been identified) are offered here and there for the original run of vehicles, which were deployed in the Madrid area by the Republicans. How many of these were captured and subsequently converted is therefore less than 5-15.
A captured MG-armed one is believed to have operated with Asensio's Column on the Madrid Front in '36.
Certainly a T-26 mod was used as the command vehicle of the armoured car group of the 'Army of the South', which otherwise had the UNL-35.
... and that's all I know.