In other words, to have a fair chance to destroy a rhino with no cover save (defining a fair chance as better than 50% chance) you would need:
5 dark lances
Or 2 meltaguns
Or 4 lascannons
To have a fairly certain chance (defining fairly certain as better than 75%) you would need:
7 dark lances
Or 5- 6 lascannons
Or still 2 meltaguns
This is all assuming the enemy vehicles never get a cover save.
This has some pretty scary implications when you consider that to really reliably down a rhino, you could need two whole ravagers for the job, and if the rhino pops smoke, even that could well not be enough.