For non-member issuer, cut the sandbox into slices according to the amount in UD, for example:
]0;10 UD]: max. 2 tx per issuer
]10 UD; 50 UD]: max. 3 tx per issuer
]50 UD; 100 UD]: max. 4 tx per issuer
]100 UD; +inf]: max. 5 tx per issuer
Each slice also has a maximum total size of : MAX (200;countTxLastBlock)
For member issuer : limit to X pending transactions per member (for example 5)