It depends on what you mean by acceptable, what's acceptable for a particular user would depend on what they are using the line for.
Or if you mean what's the maximum permitted as defined in the ITU technical recommendation documents themselves, then I suppose it would be based on the maximum allowed bit error ratio, which for VDSL2 is 1 bit error in 10
7 bits. For ADSL2/2+ it's in theory a configurable value, but I think 1 in 10
7 is the one generally used. Then, of course, the permitted numbers of CRC errors per hour depend on how many bits per second are transmitted.
There are formulae given in
Broadband Forum TR-100 and TR-114 for estimating the Bit Error Ratio from the CRC count, in the "margin verification test".
With G.INP, the ITU document mentions a minimum MTBE of 14,400 seconds, which is one uncorrected error every 4 hours, which is what is considered tolerable for watching HDTV (as in live multicast IPTV, not watching iPlayer on demand). I don't think that's the minimum MTBE actually allowed, but it's used to define the SNRM.