Each interface has a limit of around 255 inline vlan pairs. The IDSM-2 has 2 monitoring interfaces, and so has an upport limit of around 510 inline vlan pairs.

I wouldn't recommend using near that many.

The biggest limiting factors you are going to face is performance, and number of virtual sensors.

From a performance perspective the IDSM-2 is limited to around 500 Mbps regardless of how many inline vlan pairs are used.

The IDSM-2 is also limited to only 4 virtual sensors. So you can only have 4 sets of signature settings, and 4 sets of filters. So the 510 inline vlan pairs would have to be grouped within these 4 virtual sensors.