Really 40 titles limit? I have installend only 9 titles and I have low storage
The 40 titles limit is an artificial limit. Think it this way: there are two limits, a "real" limit and an artificial one.
The "real" limit is given by the (tiny, only ~140MB iirc) size of the TWLNAND. If you try to install a DSi title that requires more space that what's available in the partition, the installation will fail.
The artificial one, however, doesn't have any reason to exist other than it was put there by Nintendo. So if you try to install a DSi title, but you already have 40 DSiWare installed... well actually I don't know what
exactly happens, but it will not work. I guess it either fails to install, or it won't launch/appear on the Home menu once installed.
Now the thing is, the limits are completely separate from one another. You could have 40 installed DSiWare titles, each one occupying only, let's say, 1MB, but the installation will fail due to the artificial, 40 DSiWare titles limit. On the other hand, you could have only installed 5, really big DSiWare titles, taking up almost all the space available in the TWLNAND, and trying to install another title might fail because it would require more space than what's available, even if you're nowhere near the 40 titles limit.
Basically it's exactly what happens with 3DS titles, only that 3DS titles are installed to the SD card and the artificial limit is 300