get number block

When i use eth_getWork, i get only 3 parameters:

DATA, 32 Bytes - current block header pow-hash
DATA, 32 Bytes - the seed hash used for the DAG.
DATA, 32 Bytes - the boundary condition ("target"), 2^256 / difficulty.

How can I calculate curent block number?
eth_blockNumber - is not supported in mining
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