Question Regarding Send_to_many function of blockchain wallet api
What you need to manage is the fee rate. Bitcoin core nodes will not relay transactions (or include them in blocks) below the minimum fee rate of 1000 satoshi/kB (-minrelaytxfee), so if you keep increasing the transaction size, but not the fee, you risk not getting relayed or even mined (-blockmintxfee), since miners typically maximize the fee rate in the block being mined.