Are FPUs made of ALUs, and how many ALUs are needed to make up an FPU?
ALUs and FPUs are grossly made of the same building blocks : Adders/subtractors, multipliers, bit shifters, multiplexers...
It's just that they are organised very differently and have different widths. As FP operations are more complex, in a CPU, the FPU is usually larger than the ALU.
But :

Some processors, particularly DSPs and GPUs use the same hardware for integer and floating point instructions
There are in modern CPUs several ALUs in parallel, they are able to execute more integer than floating point instructions per cycle.
FPUs are now very, very complex and large due to the addition of SIMD instructions (MMX/AVX/SSE/NEON...).