The maximum number of transactions in a block depends on the maximum size of a block, which you can configure in MultiChain to be up to 1 GB in size. The smallest conceivable transaction (with a real signature) is about 200 bytes in size, so in theory you can fit up to five million transactions in a block. But there's no reason you'd ever want to do this, and you'd run into all sorts of large system requirements in order to process blocks of that size.
The more important statistic is how many transactions can be processed per second, independent of how they are grouped into blocks. In the case of MultiChain this is up to around 1000 tx/sec under ideal conditions on a mid-range server. More average conditions give something like 600 tx/sec.