The object recycles index numbers, so some might be tempted to look at the highest index value, but the problem with this method is that it doesn’t work in situations where, for instance, you have three blobs and one disappears.
The best way is to use the [accum] object. Attach a [t b 1] to the "touch" outlet and a [t b -1] to the "release" outlet. The bangs trigger the accum output and 1 and -1 in/decrement the accumulator (middle inlet). This will give you the number of blobs currently active.