Not sure if anyone can answer how many people are using Ethereum right now. You can monitor the network here.
There is no hardcoded block size limit in Ethereum. However, the number of contract calls and standard transactions is limited by the gas limit, which is 1.2x of the exponential moving average. So there's only so much gas that can be expended per block, even though it can grow, of course.
The current gas limit per block is at around 3 million. Note that each call to the contract to the caller cost an amount of Ether defined by
(gas sent to contract)*(price of gas as defined by caller) so sending 10 thousend transactions per block could end up quite a pricey endeavour.